Allow registering same operator schema multiple times (#18038)

Allow registering same operator schema multiple times (#18038)

Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/18038

Now that we have named overloads, we can allow registering the same function schema multiple times and just check it's identical.

This is going to be used in custom op registration since they register the schema every time a kernel is registered.

Reviewed By: dzhulgakov

Differential Revision: D14467494

fbshipit-source-id: 2c26cf72a64b65f120afe05e989302ec42597515