Support `in` membership checks (#21527)
Summary:
This PR adds support for `in` checks like `key in my_dict`
For now it leaves lists as a follow up due to the changes around `IValue` lists and it needing an `IValue` equality op.
For objects it uses the magic method `__contains__(self, key)`
](https://our.intern.facebook.com/intern/diff/15811203/)
Pull Request resolved: https://github.com/pytorch/pytorch/pull/21527
Pulled By: driazati
Differential Revision: D15811203
fbshipit-source-id: 95745060394f8a9450efaaf8ab09d9af83bea01e
