Fix handling of kwargs from common method invocations (#21499)
Summary:
When kwargs are specified in a test defined via common_method_invocations, it doesn't work if there isn't also a positional argument (`{'foo':'foo'}` without a positional arg generates a python call like: `self.method(, foo=foo)`, erroring on the `,`). I wanted to test something in a different PR and noticed I couldn't.
Also fixed some flake8 warnings I was seeing locally.
I replaced `lambda x: x` with `ident` since it seems a bit cleaner to me, but happy to revert that if others don't agree?
Pull Request resolved: https://github.com/pytorch/pytorch/pull/21499
Differential Revision: D15826974
Pulled By: nairbv
fbshipit-source-id: a3f37c80ba2303c7d9ae06241df06c7475b64e36
