Handle case when native_schema is None (#9377)

Summary:

https://fb.workplace.com/groups/pytorch.edge.users/posts/1714672769403009/?comment_id=1721960698674216

Differential Revision: D71413713

Author: metascroy

exir/program/_program.py

@@ -995,17 +995,24 @@ def keep(op):
 
         schema = op._schema
         native_schema = _pybind_schema_to_native_schema(schema)
-        if native_schema.is_mutable:
+        if native_schema is None:
             logging.warn(
-                f"Op {op} was requested for preservation by partitioner.  This request is ignored because it is mutable."
+                f"Op {op} was requested for preservation by partitioner."
+                + f"This request is ignored because Torchgen is not able to parse the schema of {op._schema}, which is necessary to determine if the op is mutable or aliases output."
             )
             return False
+        else:
+            if native_schema.is_mutable:
+                logging.warn(
+                    f"Op {op} was requested for preservation by partitioner.  This request is ignored because it is mutable."
+                )
+                return False
 
-        if native_schema.aliased_return_names() != [None]:
-            logging.warn(
-                f"Op {op} was requested for preservation by partitioner.  This request is ignored because it aliases output."
-            )
-            return False
+            if native_schema.aliased_return_names() != [None]:
+                logging.warn(
+                    f"Op {op} was requested for preservation by partitioner.  This request is ignored because it aliases output."
+                )
+                return False
 
         # Explicit block list of ops that don't work if asked for
         # preservation