Autograd using torchscript (#14604)
Summary:
This PR enables autodiff to use the forward/backward graph compiled from python code, instead of using symbolic gradients(modifying the original graph directly).
We put the map in a separate .h file for now to wait for the native_functions.yaml and derivatives.yaml merge. This should ideally go into native_functions.yaml eventually.
This PR should be enough to unblock us for now, we can start writing gradients for aten functions in python.
Differential Revision: D13494635
Pulled By: ailzhang
fbshipit-source-id: f8d51a15243ac46afd09d930c573ccdfcd9fdaaf
