fix autograd support for torch.mean(tensor, dimname) (#29199)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/29199
Previously, we called `native::mean_cpu_gpu` inside `mean(Tensor, Dimname)`;
`native::mean_cpu_gpu` is not supported by autograd. This PR replaces
`native::mean_cpu_gpu` with `at::mean(Tensor, int)` so that the dimname
overload can piggyback off of autograd support for `at::mean(Tensor,
int)`.
Also added tests (those didn't exist before) for autograd support for
named tensor reduction functions.
Test Plan: - `python test/test_namedtensor.py -v`
Differential Revision: D18334617
Pulled By: zou3519
fbshipit-source-id: 1714eb3fd93714fe860f208831e8d910f01c1c78
