Describe the bug

A dataclass in "next" doesn't get copied over in step_mdp.

To Reproduce

import dataclasses
from tensordict.tensordict import TensorDict
import torch
from torchrl.envs import step_mdp


@dataclasses.dataclass
class Test:
    a: int


dataclass = Test(1)
new_tensordict = step_mdp(
    TensorDict(
        {"next": {"state": torch.tensor([1.0]), "test": dataclass}}, batch_size=1
    )
)
print(new_tensordict.keys())
>> _StringKeys({'state': tensor([1.])})

Expected behavior

"test" in the example should be copied over.

System info

Using version 0.3.0 for both tensordict and torchrl.

Checklist

• I have checked that there is no similar issue in the repo (required)
• I have read the documentation (required)
• I have provided a minimal working example to reproduce the bug (required)