[1/n] instanceof instead of singleton for ph check (#102008)
Summary: Change placeholder check from singleton to instanceof PHBase so you can create your own PH class with metadata
Test Plan: added unit test
Reviewed By: joshuadeng
Differential Revision: D46085128
Pull Request resolved: https://github.com/pytorch/pytorch/pull/102008
Approved by: https://github.com/PaliC
