Let RpcAgent::send() return JitFuture (#49906)

Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/49906

This commit modifies RPC Message to inherit from `torch::CustomClassHolder`,
and wraps a Message in an IValue in `RpcAgent::send()`.

Test Plan: Imported from OSS

Reviewed By: lw

Differential Revision: D25719518

Pulled By: mrshenli

fbshipit-source-id: 694e40021e49e396da1620a2f81226522341550b

torch/csrc/distributed/rpc/py_rref.cpp

@@ -331,10 +331,10 @@ void PyRRef::backward(
 
     // Invoke distributed backward remotely.
     auto rpcAgent = rpc::RpcAgent::getCurrentRpcAgent();
-    rpcAgent
-        ->send(
+    rpc::RpcAgent::toFutureMessage(
+        rpcAgent->send(
             rpcAgent->getWorkerInfo(rref->owner()),
-            std::move(rrefBackwardReq).toMessage())
+            std::move(rrefBackwardReq).toMessage()))
         ->wait();
   }
 }