Support dim=None for argmax and argmin (#18264)
Summary:
Fixes: https://github.com/pytorch/pytorch/issues/18263
cc: houseroad
Pull Request resolved: https://github.com/pytorch/pytorch/pull/18264
Reviewed By: ezyang
Differential Revision: D14559234
Pulled By: houseroad
fbshipit-source-id: c5b8623752d6c6af41c6d715fd9585a65294868d
