Enable torch.cuda.amp typechecking (#45480)
Summary:
Fix `torch._C._autocast_*_nesting` declarations in __init__.pyi
Fix iterable constructor logic: not every iterable can be constructed using `type(val)(val)` trick, for example it would not work for `val=range(10)` although `isinstance(val, Iterable)` is True
Change optional resolution logic to meet mypy expectations
Fixes https://github.com/pytorch/pytorch/issues/45436
Pull Request resolved: https://github.com/pytorch/pytorch/pull/45480
Reviewed By: walterddr
Differential Revision: D23982822
Pulled By: malfet
fbshipit-source-id: 6418a28d04ece1b2427dcde4b71effb67856a872
