Fix pack_segments backward when grad is non-contig

[spcyppt]

Summary: X-link: https://github.com/facebookresearch/FBGEMM/pull/99

pack_segments backward ops works incorrectly, i.e., the input's gradient will be incorrect when the backward input grad_out is not contiguous.

This diff ensures the gradients are contiguous. Unit test is also added.

https://fb.workplace.com/groups/2126278550786248/posts/8368626576551383/

Differential Revision: D61314676

[facebook-github-bot]

This pull request was exported from Phabricator. Differential Revision: D61314676

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[facebook-github-bot]

This pull request was exported from Phabricator. Differential Revision: D61314676

[facebook-github-bot]

This pull request was exported from Phabricator. Differential Revision: D61314676

[facebook-github-bot]

This pull request was exported from Phabricator. Differential Revision: D61314676

[facebook-github-bot]

This pull request was exported from Phabricator. Differential Revision: D61314676

[facebook-github-bot]

This pull request was exported from Phabricator. Differential Revision: D61314676

[facebook-github-bot]

This pull request has been merged in pytorch/FBGEMM@81efd37168039997d84bfc0134e1333e2140bc98.

[facebook-github-bot]

This pull request has been reverted by a4a66611e951b4c4790cb0e1c37aa3232876f9e3.