modify `select_equals_backward` to propage only to a single value (#36316)
Summary:
Renames `select_equals_backward` to `select_first_equal_backward` and makes sure it propagates to a single value.
Fixes [https://github.com/pytorch/pytorch/issues/35699](https://github.com/pytorch/pytorch/issues/35699).
Pull Request resolved: https://github.com/pytorch/pytorch/pull/36316
Differential Revision: D21403848
Pulled By: albanD
fbshipit-source-id: b260cd79289162ee5733887d2afe8203945baee6
