Fix deserialization of nested fqn + serialization returns fqns for pr…

…eserve_module_signature (#1906)

Summary:
Pull Request resolved: #1906

Return all sparse fqns for preserve_module_signature, ensuring validity in swapping out with eager modules in unflattened model. Also, fix bug in deserialization

Reviewed By: IvanKobzarev

Differential Revision: D56361242

fbshipit-source-id: a73966ef6426682dbca2ed2f5b9f014ea7c15c11

torchrec/ir/utils.py

@@ -5,11 +5,9 @@
 # This source code is licensed under the BSD-style license found in the
 # LICENSE file in the root directory of this source tree.
 
-# pyre-strict
-
 #!/usr/bin/env python3
 
-from typing import Type
+from typing import List, Tuple, Type
 
 import torch
 
@@ -25,19 +23,34 @@
 def serialize_embedding_modules(
     model: nn.Module,
     serializer_cls: Type[SerializerInterface] = DEFAULT_SERIALIZER_CLS,
-) -> nn.Module:
-    for _, module in model.named_modules():
+) -> Tuple[nn.Module, List[str]]:
+    """
+    Takes all the modules that are of type `serializer_cls` and serializes them
+    in the given format with a registered buffer to the module.
+
+    Returns the modified module and the list of fqns that had the buffer added.
+    """
+    preserve_fqns = []
+    for fqn, module in model.named_modules():
         if type(module).__name__ in serializer_cls.module_to_serializer_cls:
             serialized_module = serializer_cls.serialize(module)
             module.register_buffer("ir_metadata", serialized_module, persistent=False)
+            preserve_fqns.append(fqn)
 
-    return model
+    return model, preserve_fqns
 
 
 def deserialize_embedding_modules(
     ep: ExportedProgram,
     serializer_cls: Type[SerializerInterface] = DEFAULT_SERIALIZER_CLS,
 ) -> nn.Module:
+    """
+    Takes ExportedProgram (IR) and looks for ir_metadata buffer.
+    If found, deserializes the buffer and replaces the module with the deserialized
+    module.
+
+    Returns the unflattened ExportedProgram with the deserialized modules.
+    """
     model = torch.export.unflatten(ep)
     module_type_dict = {}
     for node in ep.graph.nodes:
@@ -62,6 +75,11 @@ def deserialize_embedding_modules(
             fqn_to_new_module[fqn] = deserialized_module
 
     for fqn, new_module in fqn_to_new_module.items():
-        setattr(model, fqn, new_module)
+        # handle nested attribute like "x.y.z"
+        attrs = fqn.split(".")
+        parent = model
+        for a in attrs[:-1]:
+            parent = getattr(parent, a)
+        setattr(parent, attrs[-1], new_module)
 
     return model