Add --check-untyped-defs to mypy.ini and test suite (#37594)
Summary:
Also move the ignores for imports to the bottom in `mypy.ini`, those are much less interesting - start with the stuff people want to work on.
Second commit tests the instructions: remove an ignore, fix the issue.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/37594
Differential Revision: D21434858
Pulled By: ezyang
fbshipit-source-id: 4f1a6868cdb4cb59d072bcf105f48c3a5ba3ff98
