Implement name inference for mm, addmm (#24306)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/24306
Featuring:
- a new way of writing name inference tests. At some point I'll migrate
the older tests over.
- The out= variants aren't implemented. This is because they are a
little weird: the output gets resized, but I haven't throught through
what semantics that should have.
Test Plan: - new tests [namedtensor ci]
Differential Revision: D16915801
Pulled By: zou3519
fbshipit-source-id: 29ae2ee414c7d98e042965458c5dccef7ddbd4dd
