[jit] avoid pre-convert tensor to cpu in pickling (#38898)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/38898
Pickling will pickle the tensor meta info, and its up to the jit
exporter or other upstream who use the pickler to decide how to write
the actual tensor data.
This PR make we call getWritableTensorData in upper level so that rpc
and TensorPipe can leverge it with only pickling tensor meta data without
converting the tensor from GPU to CPU.
Test Plan: Imported from OSS
Differential Revision: D21879866
Pulled By: wanchaol
fbshipit-source-id: 75f7ff4073e4ad15b6588973dcbdc48f97a8329f
