[monarch] implement __len__ for MeshTrait

straightforward enough

Differential Revision: [D79483634](https://our.internmc.facebook.com/intern/diff/D79483634/)

ghstack-source-id: 300251671
Pull Request resolved: #729

Author: suo

python/monarch/_src/actor/actor_mesh.py

@@ -444,9 +444,6 @@ def items(self) -> Iterable[Tuple[Point, R]]:
     def __iter__(self) -> Iterator[Tuple[Point, R]]:
         return iter(self.items())
 
-    def __len__(self) -> int:
-        return len(self._shape)
-
     def __repr__(self) -> str:
         return f"ValueMesh({self._shape})"
 

python/monarch/_src/actor/shape.py

@@ -224,5 +224,8 @@ def size(self, dim: Union[None, str, Sequence[str]] = None) -> int:
     def sizes(self) -> dict[str, int]:
         return dict(zip(self._labels, self._ndslice.sizes))
 
+    def __len__(self) -> int:
+        return len(self._ndslice)
+
 
 __all__ = ["NDSlice", "Shape", "MeshTrait"]

python/tests/test_python_actors.py

@@ -598,3 +598,9 @@ def test_ported_actor():
     proc_mesh = local_proc_mesh(gpus=1).get()
     a = proc_mesh.spawn("port_actor", PortedActor).get()
     assert 5 == a.add.call_one(2).get()
+
+
+def test_mesh_len():
+    proc_mesh = local_proc_mesh(gpus=12).get()
+    s = proc_mesh.spawn("sync_actor", SyncActor).get()
+    assert 12 == len(s)