Add rpc.functions.async_execution decorator for rpc_sync/rpc_async (#39216)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/39216
The `rpc.functions.async_execution` decorator specifies that the
wrapped function is guaranteed to return a `torch.futures.Future`.
The decorator adds a `_wrapped_async_rpc_function` attribute to
the wrapper function. The caller retrieves this information and
then sets `isAsyncFunction` argument accordingly which is later
added to PythonCall RPC message as a field. On the callee side,
if the PythonCall carries an asynchronous function, it will cast
the function's return value to a jit::PythonFutureWrapper object,
and then install response creation and communication as a callback
on the that jit::PythonFutureWrapper.
For applications, this feature is useful when a function needs to
wait for IO or additional singaling. In those cases, marking the
user function as `rpc.functions.async_execution` will prevent it
from blocking one thread on callee for too long.
Test Plan: Imported from OSS
Reviewed By: rohan-varma
Differential Revision: D21779962
fbshipit-source-id: 6b6aa698bf6f91dad6ed2a7ee433df429b59e941
