Hooks for C++ API (#24393)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/24393
Ability to register hook on a variable, similar to python autograd API. register_hook will take a function as argument and create a CppFunctionPreHook similar to PyFunctionPreHook.
It will return the index of the hook which can be passed to remove_hook to disable the hook.
Test Plan: Added tests.
Differential Revision: D16861722
fbshipit-source-id: d08047f932e38c7bde04283a18b2d0311c8ad604
