More and more utils files are going to come up (hopefully not too many), so it's time to make an utils folder. It seemed overkill at the time but it would have be better.

The functions are also migrated into more meaningful files (torch_utils.py contains all the torch related utils now).

All the changes are done to maximize backward compatibility (BC) but I could not reach it perfectly.

Things which didn't change

from asteroid.utils import prepare_parser_from_dict
from asteroid.utils import tensors_to_device # Changed but accessible
from asteroid import torch_utils  # global asteroid import

What did change

As torch_utils.py is imported in asteroid/__init__.py for BC, former function from the asteroid/torch_utils.py "package" cannot be imported as follows

from asteroid.torch_utils import pad_x_to_y   # this breaks

Replace by

from asteroid import torch_utils 
torch_utils.pad_x_to_y(...)  # this works.