Clean up mode handling in python dispatcher (#121083)
Things that were bad before this PR:
1. Temporarily unsetting functional tensor mode and proxy mode both had duplicate implementation
2. There are variants of mode handling private utils that has duplicate implementation. (different APIs calling repeated implementation, so i refactored)
3. _push_mode API used to take dispatch key argument which is not necessary.
4. There are unused APIs.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/121083
Approved by: https://github.com/zou3519
