Helper to freeze specific model layers

[alexander-soare]

Currently in draft for visibility.

Adds freeze/unfreeze to timm.utils. These let you provide a list of modules or module names for which to freeze parameters.

• Resolve the ugliness of what to do if user provides a batch norm layer directly. Right now this won't always work because we need to do convert it in place within the scope of the parent module. But we can't do this if we aren't explicitly given the parent module.
• Implement unfreezing of batch norm layers.
  • @rwightman any idea on this one? I would consider doing in place replacement of all FrozenBatchNorm2d layers to BatchNorm2d (so the exact reverse of FrozenBatchNorm2d.convert_frozen_batchnorm).

[alexander-soare]

Will hopefully pick this up again next week!

[alexander-soare]

@rwightman ready to review

The only thing kind of bothering me about this one is that there is a BatchNorm1d in levit and the freeze only handles BatchNorm2d. Wasn't familiar enough with levit to know if that case needs to be handled. In any case, the docstrings are pretty clear that only BatchNorm2d is handled.

[rwightman]

@alexander-soare thanks! I tested witha few models today, looks good