Consolidate builtin/python_udf RPC to return ivalue::Future like torchscript RPC does (#35154)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/35154
This is for issue https://github.com/pytorch/pytorch/issues/34999.
close https://github.com/pytorch/pytorch/issues/34999.
https://github.com/pytorch/pytorch/issues/34997 need more work.
This will make a few work items easier, like 1) Dist autograd profiler, 2) JIT annotation for Future.
Test Plan:
```
buck test mode/dev-nosan //caffe2/test/distributed/rpc:rpc_fork
buck test mode/dev-nosan //caffe2/test/distributed/rpc:rpc_fork -- test_rref_forward_chain --stress-runs 100
buck build mode/dev-nosan //caffe2/test/distributed/rpc:rpc_fork && \
buck-out/gen/caffe2/test/distributed/rpc/rpc_fork\#binary.par \
-r test_call_method_on_rref
```
buck test mode/dev-nosan //caffe2/test/distributed/rpc:rpc_fork -- 'test_rref_proxy_class \(fb\.test_rpc_fork\.RpcTestWithFork\)' --stress-runs 100
test_rref_proxy_reuse
test_handle_send_exceptions
```
buck test mode/dev-nosan //caffe2/test/distributed/rpc/jit:rpc_fork
buck build mode/dev-nosan //caffe2/test/distributed/rpc/jit:rpc_fork && \
buck-out/gen/caffe2/test/distributed/rpc/jit/rpc_fork\#binary.par \
-r test_script_call_python_return_future
```
Differential Revision: D7722184
fbshipit-source-id: bd92b855bfea4913d6672700590c57622fa86e0e
