Name inference rule for mean, std, var, std_mean, var_mean (#25431)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/25431
I put the name propagation logic in a central place, `make_reduction`,
that creates a TensorIterator for the reduction. This lets us implement
name inference rules for mean, std, var, std_mean, and var_mean.
Test Plan
- new tests [namedtensor ci]
Test Plan: Imported from OSS
Differential Revision: D17123577
Pulled By: zou3519
fbshipit-source-id: 2d47080a40da0c4bcabbb3df71ffa8fbeb7a14c6
