[JIT x RPC] Consolidate Future type class and Future impl class (#40406)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/40406
Same motivation for https://github.com/pytorch/pytorch/issues/35110.
`Future` and `RRef` are two important types for `rpc` module, should make users feel easy to use.
Reference, https://www.sphinx-doc.org/en/master/usage/extensions/autodoc.html#directive-autoclass
Follow https://github.com/pytorch/pytorch/pull/35694.
ghstack-source-id: 106484664
Test Plan:
```
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_rref_local_value
```
```
buck test mode/dev-nosan //caffe2/test/distributed/rpc/tensorpipe:rpc_fork_tensorpipe
```
pyre -l caffe2/torch/fb/training_toolkit
pyre -l caffe2/torch/fb/distributed
pyre -l aiplatform
Differential Revision: D7722176
fbshipit-source-id: f3b9ccd7bccb233b2b33ad59dd65e178ba34d67f
