bind autograd.grad function into TorchScript (#24871)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/24871
Bind the torch.autograd.grad function into TorchScript so that well
formed inputs can directly call this from a TorchScript function.
This also change the serliazation a bit, it fixes a small bug where node
output type can never be tensor type in prim::ListConstruct(only its elementype can be), and add the case where we need to annotate the ListType if the element type is optional type to preserve type information when reimport
Differential Revision: D16923273
fbshipit-source-id: 151cc13411c8c287def35b4e65122d9fc083ccfd
