[RELAND2] Change AccumulateGrad to yield `.grad`s that match weights' memory layout (#40358)
Summary:
https://github.com/pytorch/pytorch/pull/40129 fixed the error responsible for the first revert, but exposed another error in the same test.
This PR is intended as the "master copy" for merge, and it runs on full CI.
Two other PRs (restricted to run on a small subset of CI) supporting debugging DDP failures/hangs with multiple devices per process (`test_c10d.py:DistributedDataParallelTest.test_grad_layout_1devicemodule_2replicaperprocess`).
- https://github.com/pytorch/pytorch/pull/40290 tries the test with purely rowmajor contiguous params on an untouched master. In other words https://github.com/pytorch/pytorch/pull/40290 contains none of this PR's diffs aside from the test itself.
- https://github.com/pytorch/pytorch/pull/40178, for comparison, tries the test with this PR's diffs.
Both fail the same way, indicating failure is unrelated to this PR's other diffs.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/40358
Differential Revision: D22165785
Pulled By: albanD
fbshipit-source-id: ac7cdd79af5c080ab74341671392dca8e717554e
