mmditx.py

### This file contains impls for MM-DiT, the core model component of SD3

import math
from typing import Dict, List, Optional

import numpy as np
import torch
import torch.nn as nn
from einops import rearrange, repeat

from other_impls import Mlp, attention


class PatchEmbed(nn.Module):
    """2D Image to Patch Embedding"""

    def __init__(
        self,
        img_size: Optional[int] = 224,
        patch_size: int = 16,
        in_chans: int = 3,
        embed_dim: int = 768,
        flatten: bool = True,
        bias: bool = True,
        strict_img_size: bool = True,
        dynamic_img_pad: bool = False,
        dtype=None,
        device=None,
    ):
        super().__init__()
        self.patch_size = (patch_size, patch_size)
        if img_size is not None:
            self.img_size = (img_size, img_size)
            self.grid_size = tuple(
                [s // p for s, p in zip(self.img_size, self.patch_size)]
            )
            self.num_patches = self.grid_size[0] * self.grid_size[1]
        else:
            self.img_size = None
            self.grid_size = None
            self.num_patches = None

        # flatten spatial dim and transpose to channels last, kept for bwd compat
        self.flatten = flatten
        self.strict_img_size = strict_img_size
        self.dynamic_img_pad = dynamic_img_pad

        self.proj = nn.Conv2d(
            in_chans,
            embed_dim,
            kernel_size=patch_size,
            stride=patch_size,
            bias=bias,
            dtype=dtype,
            device=device,
        )

    def forward(self, x):
        B, C, H, W = x.shape
        x = self.proj(x)
        if self.flatten:
            x = x.flatten(2).transpose(1, 2)  # NCHW -> NLC
        return x


def modulate(x, shift, scale):
    if shift is None:
        shift = torch.zeros_like(scale)
    return x * (1 + scale.unsqueeze(1)) + shift.unsqueeze(1)


#################################################################################
#                   Sine/Cosine Positional Embedding Functions                  #
#################################################################################


def get_2d_sincos_pos_embed(
    embed_dim,
    grid_size,
    cls_token=False,
    extra_tokens=0,
    scaling_factor=None,
    offset=None,
):
    """
    grid_size: int of the grid height and width
    return:
    pos_embed: [grid_size*grid_size, embed_dim] or [1+grid_size*grid_size, embed_dim] (w/ or w/o cls_token)
    """
    grid_h = np.arange(grid_size, dtype=np.float32)
    grid_w = np.arange(grid_size, dtype=np.float32)
    grid = np.meshgrid(grid_w, grid_h)  # here w goes first
    grid = np.stack(grid, axis=0)
    if scaling_factor is not None:
        grid = grid / scaling_factor
    if offset is not None:
        grid = grid - offset
    grid = grid.reshape([2, 1, grid_size, grid_size])
    pos_embed = get_2d_sincos_pos_embed_from_grid(embed_dim, grid)
    if cls_token and extra_tokens > 0:
        pos_embed = np.concatenate(
            [np.zeros([extra_tokens, embed_dim]), pos_embed], axis=0
        )
    return pos_embed


def get_2d_sincos_pos_embed_from_grid(embed_dim, grid):
    assert embed_dim % 2 == 0
    # use half of dimensions to encode grid_h
    emb_h = get_1d_sincos_pos_embed_from_grid(embed_dim // 2, grid[0])  # (H*W, D/2)
    emb_w = get_1d_sincos_pos_embed_from_grid(embed_dim // 2, grid[1])  # (H*W, D/2)
    emb = np.concatenate([emb_h, emb_w], axis=1)  # (H*W, D)
    return emb


def get_1d_sincos_pos_embed_from_grid(embed_dim, pos):
    """
    embed_dim: output dimension for each position
    pos: a list of positions to be encoded: size (M,)
    out: (M, D)
    """
    assert embed_dim % 2 == 0
    omega = np.arange(embed_dim // 2, dtype=np.float64)
    omega /= embed_dim / 2.0
    omega = 1.0 / 10000**omega  # (D/2,)
    pos = pos.reshape(-1)  # (M,)
    out = np.einsum("m,d->md", pos, omega)  # (M, D/2), outer product
    emb_sin = np.sin(out)  # (M, D/2)
    emb_cos = np.cos(out)  # (M, D/2)
    return np.concatenate([emb_sin, emb_cos], axis=1)  # (M, D)


#################################################################################
#               Embedding Layers for Timesteps and Class Labels                 #
#################################################################################


class TimestepEmbedder(nn.Module):
    """Embeds scalar timesteps into vector representations."""

    def __init__(
        self, hidden_size, frequency_embedding_size=256, dtype=None, device=None
    ):
        super().__init__()
        self.mlp = nn.Sequential(
            nn.Linear(
                frequency_embedding_size,
                hidden_size,
                bias=True,
                dtype=dtype,
                device=device,
            ),
            nn.SiLU(),
            nn.Linear(hidden_size, hidden_size, bias=True, dtype=dtype, device=device),
        )
        self.frequency_embedding_size = frequency_embedding_size

    @staticmethod
    def timestep_embedding(t, dim, max_period=10000):
        """
        Create sinusoidal timestep embeddings.
        :param t: a 1-D Tensor of N indices, one per batch element.
                          These may be fractional.
        :param dim: the dimension of the output.
        :param max_period: controls the minimum frequency of the embeddings.
        :return: an (N, D) Tensor of positional embeddings.
        """
        half = dim // 2
        freqs = torch.exp(
            -math.log(max_period)
            * torch.arange(start=0, end=half, dtype=torch.float32)
            / half
        ).to(device=t.device)
        args = t[:, None].float() * freqs[None]
        embedding = torch.cat([torch.cos(args), torch.sin(args)], dim=-1)
        if dim % 2:
            embedding = torch.cat(
                [embedding, torch.zeros_like(embedding[:, :1])], dim=-1
            )
        if torch.is_floating_point(t):
            embedding = embedding.to(dtype=t.dtype)
        return embedding

    def forward(self, t, dtype, **kwargs):
        t_freq = self.timestep_embedding(t, self.frequency_embedding_size).to(dtype)
        t_emb = self.mlp(t_freq)
        return t_emb


class VectorEmbedder(nn.Module):
    """Embeds a flat vector of dimension input_dim"""

    def __init__(self, input_dim: int, hidden_size: int, dtype=None, device=None):
        super().__init__()
        self.mlp = nn.Sequential(
            nn.Linear(input_dim, hidden_size, bias=True, dtype=dtype, device=device),
            nn.SiLU(),
            nn.Linear(hidden_size, hidden_size, bias=True, dtype=dtype, device=device),
        )

    def forward(self, x: torch.Tensor) -> torch.Tensor:
        return self.mlp(x)


#################################################################################
#                                 Core DiT Model                                #
#################################################################################


def split_qkv(qkv, head_dim):
    qkv = qkv.reshape(qkv.shape[0], qkv.shape[1], 3, -1, head_dim).movedim(2, 0)
    return qkv[0], qkv[1], qkv[2]


def optimized_attention(qkv, num_heads):
    return attention(qkv[0], qkv[1], qkv[2], num_heads)


class SelfAttention(nn.Module):
    ATTENTION_MODES = ("xformers", "torch", "torch-hb", "math", "debug")

    def __init__(
        self,
        dim: int,
        num_heads: int = 8,
        qkv_bias: bool = False,
        qk_scale: Optional[float] = None,
        attn_mode: str = "xformers",
        pre_only: bool = False,
        qk_norm: Optional[str] = None,
        rmsnorm: bool = False,
        dtype=None,
        device=None,
    ):
        super().__init__()
        self.num_heads = num_heads
        self.head_dim = dim // num_heads

        self.qkv = nn.Linear(dim, dim * 3, bias=qkv_bias, dtype=dtype, device=device)
        if not pre_only:
            self.proj = nn.Linear(dim, dim, dtype=dtype, device=device)
        assert attn_mode in self.ATTENTION_MODES
        self.attn_mode = attn_mode
        self.pre_only = pre_only

        if qk_norm == "rms":
            self.ln_q = RMSNorm(
                self.head_dim,
                elementwise_affine=True,
                eps=1.0e-6,
                dtype=dtype,
                device=device,
            )
            self.ln_k = RMSNorm(
                self.head_dim,
                elementwise_affine=True,
                eps=1.0e-6,
                dtype=dtype,
                device=device,
            )
        elif qk_norm == "ln":
            self.ln_q = nn.LayerNorm(
                self.head_dim,
                elementwise_affine=True,
                eps=1.0e-6,
                dtype=dtype,
                device=device,
            )
            self.ln_k = nn.LayerNorm(
                self.head_dim,
                elementwise_affine=True,
                eps=1.0e-6,
                dtype=dtype,
                device=device,
            )
        elif qk_norm is None:
            self.ln_q = nn.Identity()
            self.ln_k = nn.Identity()
        else:
            raise ValueError(qk_norm)

    def pre_attention(self, x: torch.Tensor):
        B, L, C = x.shape
        qkv = self.qkv(x)
        q, k, v = split_qkv(qkv, self.head_dim)
        q = self.ln_q(q).reshape(q.shape[0], q.shape[1], -1)
        k = self.ln_k(k).reshape(q.shape[0], q.shape[1], -1)
        return (q, k, v)

    def post_attention(self, x: torch.Tensor) -> torch.Tensor:
        assert not self.pre_only
        x = self.proj(x)
        return x

    def forward(self, x: torch.Tensor) -> torch.Tensor:
        (q, k, v) = self.pre_attention(x)
        x = attention(q, k, v, self.num_heads)
        x = self.post_attention(x)
        return x


class RMSNorm(torch.nn.Module):
    def __init__(
        self,
        dim: int,
        elementwise_affine: bool = False,
        eps: float = 1e-6,
        device=None,
        dtype=None,
    ):
        """
        Initialize the RMSNorm normalization layer.
        Args:
            dim (int): The dimension of the input tensor.
            eps (float, optional): A small value added to the denominator for numerical stability. Default is 1e-6.
        Attributes:
            eps (float): A small value added to the denominator for numerical stability.
            weight (nn.Parameter): Learnable scaling parameter.
        """
        super().__init__()
        self.eps = eps
        self.learnable_scale = elementwise_affine
        if self.learnable_scale:
            self.weight = nn.Parameter(torch.empty(dim, device=device, dtype=dtype))
        else:
            self.register_parameter("weight", None)

    def _norm(self, x):
        """
        Apply the RMSNorm normalization to the input tensor.
        Args:
            x (torch.Tensor): The input tensor.
        Returns:
            torch.Tensor: The normalized tensor.
        """
        return x * torch.rsqrt(x.pow(2).mean(-1, keepdim=True) + self.eps)

    def forward(self, x):
        """
        Forward pass through the RMSNorm layer.
        Args:
            x (torch.Tensor): The input tensor.
        Returns:
            torch.Tensor: The output tensor after applying RMSNorm.
        """
        x = self._norm(x)
        if self.learnable_scale:
            return x * self.weight.to(device=x.device, dtype=x.dtype)
        else:
            return x


class SwiGLUFeedForward(nn.Module):
    def __init__(
        self,
        dim: int,
        hidden_dim: int,
        multiple_of: int,
        ffn_dim_multiplier: Optional[float] = None,
    ):
        """
        Initialize the FeedForward module.

        Args:
            dim (int): Input dimension.
            hidden_dim (int): Hidden dimension of the feedforward layer.
            multiple_of (int): Value to ensure hidden dimension is a multiple of this value.
            ffn_dim_multiplier (float, optional): Custom multiplier for hidden dimension. Defaults to None.

        Attributes:
            w1 (ColumnParallelLinear): Linear transformation for the first layer.
            w2 (RowParallelLinear): Linear transformation for the second layer.
            w3 (ColumnParallelLinear): Linear transformation for the third layer.

        """
        super().__init__()
        hidden_dim = int(2 * hidden_dim / 3)
        # custom dim factor multiplier
        if ffn_dim_multiplier is not None:
            hidden_dim = int(ffn_dim_multiplier * hidden_dim)
        hidden_dim = multiple_of * ((hidden_dim + multiple_of - 1) // multiple_of)

        self.w1 = nn.Linear(dim, hidden_dim, bias=False)
        self.w2 = nn.Linear(hidden_dim, dim, bias=False)
        self.w3 = nn.Linear(dim, hidden_dim, bias=False)

    def forward(self, x):
        return self.w2(nn.functional.silu(self.w1(x)) * self.w3(x))


class DismantledBlock(nn.Module):
    """A DiT block with gated adaptive layer norm (adaLN) conditioning."""

    ATTENTION_MODES = ("xformers", "torch", "torch-hb", "math", "debug")

    def __init__(
        self,
        hidden_size: int,
        num_heads: int,
        mlp_ratio: float = 4.0,
        attn_mode: str = "xformers",
        qkv_bias: bool = False,
        pre_only: bool = False,
        rmsnorm: bool = False,
        scale_mod_only: bool = False,
        swiglu: bool = False,
        qk_norm: Optional[str] = None,
        x_block_self_attn: bool = False,
        dtype=None,
        device=None,
        **block_kwargs,
    ):
        super().__init__()
        assert attn_mode in self.ATTENTION_MODES
        if not rmsnorm:
            self.norm1 = nn.LayerNorm(
                hidden_size,
                elementwise_affine=False,
                eps=1e-6,
                dtype=dtype,
                device=device,
            )
        else:
            self.norm1 = RMSNorm(hidden_size, elementwise_affine=False, eps=1e-6)
        self.attn = SelfAttention(
            dim=hidden_size,
            num_heads=num_heads,
            qkv_bias=qkv_bias,
            attn_mode=attn_mode,
            pre_only=pre_only,
            qk_norm=qk_norm,
            rmsnorm=rmsnorm,
            dtype=dtype,
            device=device,
        )
        if x_block_self_attn:
            assert not pre_only
            assert not scale_mod_only
            self.x_block_self_attn = True
            self.attn2 = SelfAttention(
                dim=hidden_size,
                num_heads=num_heads,
                qkv_bias=qkv_bias,
                attn_mode=attn_mode,
                pre_only=False,
                qk_norm=qk_norm,
                rmsnorm=rmsnorm,
                dtype=dtype,
                device=device,
            )
        else:
            self.x_block_self_attn = False
        if not pre_only:
            if not rmsnorm:
                self.norm2 = nn.LayerNorm(
                    hidden_size,
                    elementwise_affine=False,
                    eps=1e-6,
                    dtype=dtype,
                    device=device,
                )
            else:
                self.norm2 = RMSNorm(hidden_size, elementwise_affine=False, eps=1e-6)
        mlp_hidden_dim = int(hidden_size * mlp_ratio)
        if not pre_only:
            if not swiglu:
                self.mlp = Mlp(
                    in_features=hidden_size,
                    hidden_features=mlp_hidden_dim,
                    act_layer=nn.GELU(approximate="tanh"),
                    dtype=dtype,
                    device=device,
                )
            else:
                self.mlp = SwiGLUFeedForward(
                    dim=hidden_size, hidden_dim=mlp_hidden_dim, multiple_of=256
                )
        self.scale_mod_only = scale_mod_only
        if x_block_self_attn:
            assert not pre_only
            assert not scale_mod_only
            n_mods = 9
        elif not scale_mod_only:
            n_mods = 6 if not pre_only else 2
        else:
            n_mods = 4 if not pre_only else 1
        self.adaLN_modulation = nn.Sequential(
            nn.SiLU(),
            nn.Linear(
                hidden_size, n_mods * hidden_size, bias=True, dtype=dtype, device=device
            ),
        )
        self.pre_only = pre_only

    def pre_attention(self, x: torch.Tensor, c: torch.Tensor):
        assert x is not None, "pre_attention called with None input"
        if not self.pre_only:
            if not self.scale_mod_only:
                shift_msa, scale_msa, gate_msa, shift_mlp, scale_mlp, gate_mlp = (
                    self.adaLN_modulation(c).chunk(6, dim=1)
                )
            else:
                shift_msa = None
                shift_mlp = None
                scale_msa, gate_msa, scale_mlp, gate_mlp = self.adaLN_modulation(
                    c
                ).chunk(4, dim=1)
            qkv = self.attn.pre_attention(modulate(self.norm1(x), shift_msa, scale_msa))
            return qkv, (x, gate_msa, shift_mlp, scale_mlp, gate_mlp)
        else:
            if not self.scale_mod_only:
                shift_msa, scale_msa = self.adaLN_modulation(c).chunk(2, dim=1)
            else:
                shift_msa = None
                scale_msa = self.adaLN_modulation(c)
            qkv = self.attn.pre_attention(modulate(self.norm1(x), shift_msa, scale_msa))
            return qkv, None

    def post_attention(self, attn, x, gate_msa, shift_mlp, scale_mlp, gate_mlp):
        assert not self.pre_only
        x = x + gate_msa.unsqueeze(1) * self.attn.post_attention(attn)
        x = x + gate_mlp.unsqueeze(1) * self.mlp(
            modulate(self.norm2(x), shift_mlp, scale_mlp)
        )
        return x

    def pre_attention_x(self, x: torch.Tensor, c: torch.Tensor) -> torch.Tensor:
        assert self.x_block_self_attn
        (
            shift_msa,
            scale_msa,
            gate_msa,
            shift_mlp,
            scale_mlp,
            gate_mlp,
            shift_msa2,
            scale_msa2,
            gate_msa2,
        ) = self.adaLN_modulation(c).chunk(9, dim=1)
        x_norm = self.norm1(x)
        qkv = self.attn.pre_attention(modulate(x_norm, shift_msa, scale_msa))
        qkv2 = self.attn2.pre_attention(modulate(x_norm, shift_msa2, scale_msa2))
        return (
            qkv,
            qkv2,
            (
                x,
                gate_msa,
                shift_mlp,
                scale_mlp,
                gate_mlp,
                gate_msa2,
            ),
        )

    def post_attention_x(
        self,
        attn,
        attn2,
        x,
        gate_msa,
        shift_mlp,
        scale_mlp,
        gate_mlp,
        gate_msa2,
        attn1_dropout: float = 0.0,
    ):
        assert not self.pre_only
        if attn1_dropout > 0.0:
            # Use torch.bernoulli to implement dropout, only dropout the batch dimension
            attn1_dropout = torch.bernoulli(
                torch.full((attn.size(0), 1, 1), 1 - attn1_dropout, device=attn.device)
            )
            attn_ = (
                gate_msa.unsqueeze(1) * self.attn.post_attention(attn) * attn1_dropout
            )
        else:
            attn_ = gate_msa.unsqueeze(1) * self.attn.post_attention(attn)
        x = x + attn_
        attn2_ = gate_msa2.unsqueeze(1) * self.attn2.post_attention(attn2)
        x = x + attn2_
        mlp_ = gate_mlp.unsqueeze(1) * self.mlp(
            modulate(self.norm2(x), shift_mlp, scale_mlp)
        )
        x = x + mlp_
        return x, (gate_msa, gate_msa2, gate_mlp, attn_, attn2_)

    def forward(self, x: torch.Tensor, c: torch.Tensor) -> torch.Tensor:
        assert not self.pre_only
        if self.x_block_self_attn:
            (q, k, v), (q2, k2, v2), intermediates = self.pre_attention_x(x, c)
            attn = attention(q, k, v, self.attn.num_heads)
            attn2 = attention(q2, k2, v2, self.attn2.num_heads)
            return self.post_attention_x(attn, attn2, *intermediates)
        else:
            (q, k, v), intermediates = self.pre_attention(x, c)
            attn = attention(q, k, v, self.attn.num_heads)
            return self.post_attention(attn, *intermediates)


def block_mixing(context, x, context_block, x_block, c):
    assert context is not None, "block_mixing called with None context"
    context_qkv, context_intermediates = context_block.pre_attention(context, c)

    if x_block.x_block_self_attn:
        x_qkv, x_qkv2, x_intermediates = x_block.pre_attention_x(x, c)
    else:
        x_qkv, x_intermediates = x_block.pre_attention(x, c)

    o = []
    for t in range(3):
        o.append(torch.cat((context_qkv[t], x_qkv[t]), dim=1))
    q, k, v = tuple(o)

    attn = attention(q, k, v, x_block.attn.num_heads)
    context_attn, x_attn = (
        attn[:, : context_qkv[0].shape[1]],
        attn[:, context_qkv[0].shape[1] :],
    )

    if not context_block.pre_only:
        context = context_block.post_attention(context_attn, *context_intermediates)
    else:
        context = None

    if x_block.x_block_self_attn:
        x_q2, x_k2, x_v2 = x_qkv2
        attn2 = attention(x_q2, x_k2, x_v2, x_block.attn2.num_heads)
    else:
        x = x_block.post_attention(x_attn, *x_intermediates)

    return context, x


class JointBlock(nn.Module):
    """just a small wrapper to serve as a fsdp unit"""

    def __init__(self, *args, **kwargs):
        super().__init__()
        pre_only = kwargs.pop("pre_only")
        qk_norm = kwargs.pop("qk_norm", None)
        x_block_self_attn = kwargs.pop("x_block_self_attn", False)
        self.context_block = DismantledBlock(
            *args, pre_only=pre_only, qk_norm=qk_norm, **kwargs
        )
        self.x_block = DismantledBlock(
            *args,
            pre_only=False,
            qk_norm=qk_norm,
            x_block_self_attn=x_block_self_attn,
            **kwargs,
        )

    def forward(self, *args, **kwargs):
        return block_mixing(
            *args, context_block=self.context_block, x_block=self.x_block, **kwargs
        )


class FinalLayer(nn.Module):
    """
    The final layer of DiT.
    """

    def __init__(
        self,
        hidden_size: int,
        patch_size: int,
        out_channels: int,
        total_out_channels: Optional[int] = None,
        dtype=None,
        device=None,
    ):
        super().__init__()
        self.norm_final = nn.LayerNorm(
            hidden_size, elementwise_affine=False, eps=1e-6, dtype=dtype, device=device
        )
        self.linear = (
            nn.Linear(
                hidden_size,
                patch_size * patch_size * out_channels,
                bias=True,
                dtype=dtype,
                device=device,
            )
            if (total_out_channels is None)
            else nn.Linear(
                hidden_size, total_out_channels, bias=True, dtype=dtype, device=device
            )
        )
        self.adaLN_modulation = nn.Sequential(
            nn.SiLU(),
            nn.Linear(
                hidden_size, 2 * hidden_size, bias=True, dtype=dtype, device=device
            ),
        )

    def forward(self, x: torch.Tensor, c: torch.Tensor) -> torch.Tensor:
        shift, scale = self.adaLN_modulation(c).chunk(2, dim=1)
        x = modulate(self.norm_final(x), shift, scale)
        x = self.linear(x)
        return x


class MMDiTX(nn.Module):
    """Diffusion model with a Transformer backbone."""

    def __init__(
        self,
        input_size: int = 32,
        patch_size: int = 2,
        in_channels: int = 4,
        depth: int = 28,
        mlp_ratio: float = 4.0,
        learn_sigma: bool = False,
        adm_in_channels: Optional[int] = None,
        context_embedder_config: Optional[Dict] = None,
        register_length: int = 0,
        attn_mode: str = "torch",
        rmsnorm: bool = False,
        scale_mod_only: bool = False,
        swiglu: bool = False,
        out_channels: Optional[int] = None,
        pos_embed_scaling_factor: Optional[float] = None,
        pos_embed_offset: Optional[float] = None,
        pos_embed_max_size: Optional[int] = None,
        num_patches=None,
        qk_norm: Optional[str] = None,
        x_block_self_attn_layers: Optional[List[int]] = [],
        qkv_bias: bool = True,
        dtype=None,
        device=None,
        verbose=False,
    ):
        super().__init__()
        if verbose:
            print(
                f"mmdit initializing with: {input_size=}, {patch_size=}, {in_channels=}, {depth=}, {mlp_ratio=}, {learn_sigma=}, {adm_in_channels=}, {context_embedder_config=}, {register_length=}, {attn_mode=}, {rmsnorm=}, {scale_mod_only=}, {swiglu=}, {out_channels=}, {pos_embed_scaling_factor=}, {pos_embed_offset=}, {pos_embed_max_size=}, {num_patches=}, {qk_norm=}, {qkv_bias=}, {dtype=}, {device=}"
            )
        self.dtype = dtype
        self.learn_sigma = learn_sigma
        self.in_channels = in_channels
        default_out_channels = in_channels * 2 if learn_sigma else in_channels
        self.out_channels = (
            out_channels if out_channels is not None else default_out_channels
        )
        self.patch_size = patch_size
        self.pos_embed_scaling_factor = pos_embed_scaling_factor
        self.pos_embed_offset = pos_embed_offset
        self.pos_embed_max_size = pos_embed_max_size
        self.x_block_self_attn_layers = x_block_self_attn_layers

        # apply magic --> this defines a head_size of 64
        hidden_size = 64 * depth
        num_heads = depth

        self.num_heads = num_heads

        self.x_embedder = PatchEmbed(
            input_size,
            patch_size,
            in_channels,
            hidden_size,
            bias=True,
            strict_img_size=self.pos_embed_max_size is None,
            dtype=dtype,
            device=device,
        )
        self.t_embedder = TimestepEmbedder(hidden_size, dtype=dtype, device=device)

        if adm_in_channels is not None:
            assert isinstance(adm_in_channels, int)
            self.y_embedder = VectorEmbedder(
                adm_in_channels, hidden_size, dtype=dtype, device=device
            )

        self.context_embedder = nn.Identity()
        if context_embedder_config is not None:
            if context_embedder_config["target"] == "torch.nn.Linear":
                self.context_embedder = nn.Linear(
                    **context_embedder_config["params"], dtype=dtype, device=device
                )

        self.register_length = register_length
        if self.register_length > 0:
            self.register = nn.Parameter(
                torch.randn(1, register_length, hidden_size, dtype=dtype, device=device)
            )

        # num_patches = self.x_embedder.num_patches
        # Will use fixed sin-cos embedding:
        # just use a buffer already
        if num_patches is not None:
            self.register_buffer(
                "pos_embed",
                torch.zeros(1, num_patches, hidden_size, dtype=dtype, device=device),
            )
        else:
            self.pos_embed = None

        self.joint_blocks = nn.ModuleList(
            [
                JointBlock(
                    hidden_size,
                    num_heads,
                    mlp_ratio=mlp_ratio,
                    qkv_bias=qkv_bias,
                    attn_mode=attn_mode,
                    pre_only=i == depth - 1,
                    rmsnorm=rmsnorm,
                    scale_mod_only=scale_mod_only,
                    swiglu=swiglu,
                    qk_norm=qk_norm,
                    x_block_self_attn=(i in self.x_block_self_attn_layers),
                    dtype=dtype,
                    device=device,
                )
                for i in range(depth)
            ]
        )

        self.final_layer = FinalLayer(
            hidden_size, patch_size, self.out_channels, dtype=dtype, device=device
        )

    def cropped_pos_embed(self, hw):
        assert self.pos_embed_max_size is not None
        p = self.x_embedder.patch_size[0]
        h, w = hw
        # patched size
        h = h // p
        w = w // p
        assert h <= self.pos_embed_max_size, (h, self.pos_embed_max_size)
        assert w <= self.pos_embed_max_size, (w, self.pos_embed_max_size)
        top = (self.pos_embed_max_size - h) // 2
        left = (self.pos_embed_max_size - w) // 2
        spatial_pos_embed = rearrange(
            self.pos_embed,
            "1 (h w) c -> 1 h w c",
            h=self.pos_embed_max_size,
            w=self.pos_embed_max_size,
        )
        spatial_pos_embed = spatial_pos_embed[:, top : top + h, left : left + w, :]
        spatial_pos_embed = rearrange(spatial_pos_embed, "1 h w c -> 1 (h w) c")
        return spatial_pos_embed

    def unpatchify(self, x, hw=None):
        """
        x: (N, T, patch_size**2 * C)
        imgs: (N, H, W, C)
        """
        c = self.out_channels
        p = self.x_embedder.patch_size[0]
        if hw is None:
            h = w = int(x.shape[1] ** 0.5)
        else:
            h, w = hw
            h = h // p
            w = w // p
        assert h * w == x.shape[1]

        x = x.reshape(shape=(x.shape[0], h, w, p, p, c))
        x = torch.einsum("nhwpqc->nchpwq", x)
        imgs = x.reshape(shape=(x.shape[0], c, h * p, w * p))
        return imgs

    def forward_core_with_concat(
        self,
        x: torch.Tensor,
        c_mod: torch.Tensor,
        context: Optional[torch.Tensor] = None,
    ) -> torch.Tensor:
        if self.register_length > 0:
            context = torch.cat(
                (
                    repeat(self.register, "1 ... -> b ...", b=x.shape[0]),
                    context if context is not None else torch.Tensor([]).type_as(x),
                ),
                1,
            )

        # context is B, L', D
        # x is B, L, D
        for block in self.joint_blocks:
            context, x = block(context, x, c=c_mod)

        x = self.final_layer(x, c_mod)  # (N, T, patch_size ** 2 * out_channels)
        return x

    def forward(
        self,
        x: torch.Tensor,
        t: torch.Tensor,
        y: Optional[torch.Tensor] = None,
        context: Optional[torch.Tensor] = None,
    ) -> torch.Tensor:
        """
        Forward pass of DiT.
        x: (N, C, H, W) tensor of spatial inputs (images or latent representations of images)
        t: (N,) tensor of diffusion timesteps
        y: (N,) tensor of class labels
        """
        hw = x.shape[-2:]
        x = self.x_embedder(x) + self.cropped_pos_embed(hw)
        c = self.t_embedder(t, dtype=x.dtype)  # (N, D)
        if y is not None:
            y = self.y_embedder(y)  # (N, D)
            c = c + y  # (N, D)

        context = self.context_embedder(context)

        x = self.forward_core_with_concat(x, c, context)

        x = self.unpatchify(x, hw=hw)  # (N, out_channels, H, W)
        return x