Move cadence.aot.passes to cadence.aot._passes (#5921)

Summary:
Changing cadence.aot.passes to cadence.aot._passes to indicate that these passes are not covered under the API stability guarantee.

Pull Request resolved: #5921

Reviewed By: helunwencser, mcremon-meta

Differential Revision: D63926847

fbshipit-source-id: 60c4739a813422b92d2f187fceb5e62f65db6380
(cherry picked from commit 59c359f)

Author: tarun292

backends/cadence/aot/TARGETS

@@ -48,7 +48,7 @@ python_library(
 python_library(
     name = "passes",
     srcs = [
-        "passes.py",
+        "_passes.py",
     ],
     deps = [
         ":utils",

backends/cadence/aot/passes.py renamed to backends/cadence/aot/_passes.py

@@ -11,7 +11,7 @@
 
 import torch
 
-from executorch.backends.cadence.aot.passes import (
+from executorch.backends.cadence.aot._passes import (
     InitializePipeline,
     RemoveNopExpandOpPass,
     RemoveZeroSizedCatArgsPass,