Refactor select_copy into util (#2784)

Summary:
Pull Request resolved: https://github.com/pytorch/executorch/pull/2784

Refactoring the core logic of select_copy into a helper utility function so that it can be utilized by other operators that internally need to do a select operation.

Reviewed By: iseeyuan

Differential Revision: D55565485

fbshipit-source-id: 387212824c23613efe45f4be647de15763dad5aa