Add autograd hook for python rpc call (#28312)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/28312
1. currently if autograd context is valid, even tensors do not require grads and grads function are not attached.
it still send rpc with autograd meta. This is not ideal.
This diff makes some change to make sure rpc with autograd meta is sent only if autograd context is valid and tensors require grads
2. meanwhile create a utiliy to attach autograd info and functions as needed
3. add autograd send/recv functions for python rpc call
4. make changes to support nested python rpc calls
5. disallow nested dist autograd context (was landed in #27022)
ghstack-source-id: 92240367
Test Plan: unit tests
Differential Revision: D18017554
fbshipit-source-id: dbe79a5171063901a78a9b3322b9b31c159d098d
