Add CPU group_index_select fwd and bwd impl

[spcyppt]

Summary: X-link: https://github.com/facebookresearch/FBGEMM/pull/369

The GIS op interfaces are unified for all device backends, but the CPU group_index_select_dim0 is still using the decomposed version due to lack of fused impl for fwd/bwd.

This diff implements the following functions on CPU:

• group_index_select_dim0_forward_impl_cpu
• group_index_select_dim0_backward_impl_cpu

Reviewed By: q10, egienvalue

Differential Revision: D64556901

[facebook-github-bot]

This pull request was exported from Phabricator. Differential Revision: D64556901

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[facebook-github-bot]

This pull request was exported from Phabricator. Differential Revision: D64556901

[facebook-github-bot]

This pull request has been merged in pytorch/FBGEMM@fbf0096bd42a84ea01cec86edd15409c84768ac8.