Add default Saved Variable hooks (#61834)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/61834
Expose a pair of functions to Python users: torch.autograd.graph.set_saved_tensors_default_hooks(pack, unpack) and torch.autograd.graph.reset_saved_tensors_default_hooks().
These functions control the hooks applied to saved tensors: all tensors saved in that context will be packed using the pack function, then unpacked accordingly when needed.
Currently, this works by simply calling register_hooks (cf #60975) directly at the end of the constructor of a SavedVariable. This could be optimized further by not performing the copy before registering default hooks, but this would require a small refactor. Edit: the refactor is done in #61927.
A current limitation is that if users create tensors in this context, they will not be able to register additional hooks on the saved tensor.
For instance, to perform something like #28997, one could define a pack function that saves to disk whenever the tensor size is too big and returns a filename, then unpack simply reads the content of the file and outputs a tensor, e.g.:
```
def pack(x):
name = os.path.join(tmp_dir, str(uuid.uuid4()))
torch.save(x, name)
return name
def unpack(name):
return torch.load(name)
```
Test Plan: Imported from OSS
Reviewed By: zou3519
Differential Revision: D29792193
Pulled By: Varal7
fbshipit-source-id: 33e931230ef59faa3ec8b5d11ef7c05539bce77c
