[Easy][FSDP] Fix pyre error (#89930)
This PR attemps to fix the following pyre error:
```
Incompatible parameter type [6]: In call
`dist.fsdp.fully_sharded_data_parallel.FullyShardedDataParallel.__init__`,
for 7th parameter `auto_wrap_policy` expected
`Optional[typing.Callable[..., typing.Any]]` but got
`Optional[_FSDPPolicy]`.
```
Besides, this also removes the type inconsistency in code and docstring.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/89930
Approved by: https://github.com/awgu
