Rereapply optional ScalarType interface changes that were reverted in D16079809 (#22456)
Summary:
re-apply changes reverted in:
https://github.com/pytorch/pytorch/pull/22412
Also change log_softmax to take positional arguments. Long-term we do want the kwarg-only interface, but seems to currently be incompatible with jit serialization.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/22456
Differential Revision: D16097159
Pulled By: nairbv
fbshipit-source-id: 8cb73e9ca18fc66b35b873cf4a574b167a578b3d
