Add masked_index_select and refactor masked_index_put

[sryap]

Summary: This diff adds torch.ops.fbgemm.masked_index_select (will be used in subsequent diffs) and refactors torch.ops.fbgemm.masked_index_put.

• Add unit tests
• Add docstrings
• Add contiguous tensor checks

Differential Revision: D60362812

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This pull request was exported from Phabricator. Differential Revision: D60362812

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This pull request was exported from Phabricator. Differential Revision: D60362812

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This pull request was exported from Phabricator. Differential Revision: D60362812

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This pull request was exported from Phabricator. Differential Revision: D60362812

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This pull request has been merged in pytorch/FBGEMM@1fa5a68ab6c1efdf4fe8e2f7fc7bab9fce643856.