Migrate to PythonActorMesh and PythonActorMeshRef (#770)

Summary:
Pull Request resolved: #770

Pull Request resolved: #557

This diff swaps `_ActorMeshRefImpl` with `PythonActorMesh[Ref]`.

The swap itself should be straightforward since `PythonActorMesh[Ref]` should be drop-in replacements for `_ActorMeshRefImpl`. Most of the complexity in this diff is from how I tried to add a toggle between them, just in case there is any bugs with  `PythonActorMesh[Ref]`, so we can quickly switch back to `_ActorMeshRefImpl`. What I did is:

1. Add wrapper classes `EitherPyActorMesh[Ref]`, whose underlying type can be either `PythonActorMesh[Ref]` or `_ActorMeshRefImpl`;
2. a env var `USE_STANDIN_ACTOR_MESH` is used to which one would be used when instantiating `EitherPyActorMesh[Ref]`.

The landing of this diff would mean all Python-side mesh API calls should go through Rust-side's `cast` code path, except several usages of `ActorIdRef`.

Reviewed By: zdevito

Differential Revision: D78355743

fbshipit-source-id: 4aea0a8168b7dc0fff0cb24462869df0434e3ee6

Author: pzhan9

monarch_hyperactor/src/actor_mesh.rs

@@ -119,9 +119,9 @@ impl PythonActorMesh {
     }
 
     fn try_inner(&self) -> PyResult<SharedCellRef<RootActorMesh<'static, PythonActor>>> {
-        self.inner
-            .borrow()
-            .map_err(|_| PyRuntimeError::new_err("`PythonActorMesh` has already been stopped"))
+        self.inner.borrow().map_err(|_| {
+            SupervisionError::new_err("`PythonActorMesh` has already been stopped".to_string())
+        })
     }
 
     fn pickling_err(&self) -> PyErr {
@@ -144,13 +144,13 @@ impl PythonActorMesh {
         match &*unhealthy_event {
             Unhealthy::SoFarSoGood => (),
             Unhealthy::Crashed(event) => {
-                return Err(PyRuntimeError::new_err(format!(
+                return Err(SupervisionError::new_err(format!(
                     "actor mesh is unhealthy with reason: {:?}",
                     event
                 )));
             }
             Unhealthy::StreamClosed => {
-                return Err(PyRuntimeError::new_err(
+                return Err(SupervisionError::new_err(
                     "actor mesh is stopped due to proc mesh shutdown".to_string(),
                 ));
             }