[FSDP2] Removed `super().__setattr__` call (#120340)
`nn.Module.__setattr__` does not actually call `super().__setattr__()`. If we make this call in our fast path, then we will inadvertently set the parameter as an actual attribute on the module, not just as an entry in the `_parameters` dict. This can lead to a bug where after replacing the parameters on the module (e.g. via `to_empty()` from meta device), we now have both an actual attribute (old) and a new entry in `_parameters` (new). Trying to access the parameter would give the old one since Python only resolves `__getattr__` if normal attribute lookup fails.
The bug was exercised in the following PR. I wanted to land this bug fix separately.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/120340
Approved by: https://github.com/yifuwang
ghstack dependencies: #120231
