Lets @dtypes take tuples of dtypes (#36908)
Summary:
Lets dtypes take tuples of dtypes instead of just single dtypes. This pattern comes up when tests have distinct in and out types. A test in test_type_promotion is updated to use the new behavior.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/36908
Differential Revision: D21161523
Pulled By: mruberry
fbshipit-source-id: ebac81c1b6c494a2146d595fcdb3e35c22cf859c
