Add rpc.async_function decorator for TorchScript functions (#39267)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/39267
When combined with `torch.jit.script`, the order of decorators matter.
`rpc.functions.async_execution` must be the outmost one. The
`async_execution` decorator will store the TorchScript function in
attribute `_wrapped_async_rpc_function` on the wrapper function, and
pass this wrapped TorchScript function (i.e., an instance of
`torch.jit.ScriptFunction`) to RPC. The caller will mark the ScriptCall
with `isAsyncExecution=true`, and the callee will extract the returned
`Future` in C++ and install subsequent processing as a callback to
that `Future`.
Test Plan: Imported from OSS
Differential Revision: D21792688
fbshipit-source-id: de095eb148d21e9114a478e9e6047c707d34fd07
