Update fx proxy to use classmethod for __torch_function__ (#64842)

Update fx proxy to use classmethod for __torch_function__ (#64842)

Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/64842

Change the `__torch_function__` to follow best guidelines of using classmethods.
I am not sure how to handle the case where multiple tracer objects are given as input but given that before we were getting an arbitrary tracer from there based on the "self" that was arbitrarily chosen by the torch_function caller, the new implementation is not worst?
Let me know what you think!

Test Plan: Imported from OSS

Reviewed By: heitorschueroff

Differential Revision: D30991423

Pulled By: albanD

fbshipit-source-id: d28940df230b543952b278a0eb2d61cf7ae123ce