Add warning for legacy autograd function (#22922)
Summary:
When working on https://github.com/pytorch/pytorch/pull/22762, we discovered that we haven't actually deprecated legacy autograd function. This PR puts up the deprecation warning for 1.2, with the goal to remove legacy function support completely in the near future.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/22922
Differential Revision: D16363916
Pulled By: yf225
fbshipit-source-id: 4b554010a3d1f87a3fa45cc1aa29d019c8f1033c
