Enable mypy allow redefinition (#102046)
Related #101528
I tried to enable this in another PR but it uncovered a bunch of type errors: https://github.com/pytorch/pytorch/actions/runs/4999748262/jobs/8956555243?pr=101528#step:10:1305
The goal of this PR is to fix these errors.
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This PR enables [allow_redefinition = True](https://mypy.readthedocs.io/en/stable/config_file.html#confval-allow_redefinition) in `mypy.ini`, which allows for a common pattern:
> Allows variables to be redefined with an arbitrary type, as long as the redefinition is in the same block and nesting level as the original definition.
`allow_redefinition` allows mypy to be more flexible by allowing reassignment to an existing variable with a different type... for instance (from the linked PR):
https://github.com/pytorch/pytorch/blob/4a1e9230bae52aba3d0d03dc13f0416f464b2679/torch/nn/parallel/data_parallel.py#L213
A `Sequence[Union[int, torch.device]]` is narrowed to `Sequence[int]` thru reassignment to the same variable.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/102046
Approved by: https://github.com/ezyang
