Initial implementation of nanmean (#62671)

Initial implementation of nanmean (#62671)

Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/62671

Very crude first implementation of `torch.nanmean`. The current reduction kernels do not have good support for implementing nan* variants. Rather than implementing new kernels for each nan* operator, I will work on new reduction kernels with support for a `nan_policy` flag and then I will port `nanmean` to use that.

**TODO**

- [x] Fix autograd issue

Test Plan: Imported from OSS

Reviewed By: malfet

Differential Revision: D30515181

Pulled By: heitorschueroff

fbshipit-source-id: 303004ebd7ac9cf963dc4f8e2553eaded5f013f0