Fix get_unique_indices_v2 registration

[sryap]

Summary: Properly register get_unique_indices_v2 in torch.ops.fbgemm and import it in SSD-TBE.

• Use init to define and register each op only once
• Check if an op is defined before defining and registering it to avoid duplicate declaration

Differential Revision: D61294287

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This pull request was exported from Phabricator. Differential Revision: D61294287

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This pull request was exported from Phabricator. Differential Revision: D61294287

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This pull request was exported from Phabricator. Differential Revision: D61294287

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This pull request was exported from Phabricator. Differential Revision: D61294287

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This pull request has been merged in pytorch/FBGEMM@160633ca7a98c4c24ac5565f04b24eb71d1ca128.