Add assign kwarg to module.load_state_dict (#102212)
Fixes #64601 and #98906
Adds an `assign` argument to `load_state_dict` that loads params/buffers by assignment instead of doing `param.copy_(param_from_state_dict)`.
Primarily intended to remove the need for the `.to_empty()` in
```
with torch.device('meta'):
m = SomeModule()
m.to_empty()
state_dict = torch.load('...pth')
m.load_state_dict(state_dict)
```
so we can instead do
```
with torch.device('meta'):
m = SomeModule()
state_dict = torch.load('...pth')
m.load_state_dict(state_dict, assign=True)
```
**A problem with this PR for the case where the model is initialized on meta is what happens to nonpersistent buffers/params corresponding to keys missing from the state dict?**
What happens in the case where `load_state_dict(state_dict, strict=False, assign=True)` and the state_dict is missing some keys? The corresponding params missing from the `state_dict` and nonpersistent buffers would still be on `meta` and need to be manually initialized. However, I don't think we offer an API that would initialize these.
One solution would be to make these empty tensors but it might not be semantically correct...
Pull Request resolved: https://github.com/pytorch/pytorch/pull/102212
Approved by: https://github.com/albanD
