Add tests for mismatched dtypes in torch.gather. (#39689)
Summary:
https://github.com/pytorch/pytorch/pull/38646 added checks for this, but only added tets for the scatter functions.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/39689
Reviewed By: malfet
Differential Revision: D21945524
Pulled By: gchanan
fbshipit-source-id: 8b06856c06d6427b8cd929a1275422a5ed6e11cc
