Deserialize to named data store output

Pull Request resolved: #15469

Relatively easy for user to impl with the existing deserialize functionality, but think it's good to have something explicit too.
ghstack-source-id: 319841977

Differential Revision: [D83510300](https://our.internmc.facebook.com/intern/diff/D83510300/)

Author: lucylq

extension/flat_tensor/serialize/TARGETS

@@ -29,6 +29,7 @@ runtime.python_library(
     ],
     visibility = [
         "//executorch/...",
+        "@EXECUTORCH_CLIENTS",
     ],
     deps = [
         ":schema",

extension/flat_tensor/serialize/serialize.py

@@ -20,6 +20,7 @@
 from executorch.exir._serialize._cord import Cord
 from executorch.exir._serialize._dataclass import _DataclassEncoder, _json_to_dataclass
 from executorch.exir._serialize._flatbuffer import _flatc_compile, _flatc_decompile
+from executorch.exir._serialize._named_data_store import NamedDataStoreOutput
 from executorch.exir._serialize._program import _insert_flatbuffer_header
 from executorch.exir._serialize.data_serializer import (
     DataEntry,
@@ -389,6 +390,8 @@ def serialize(
     def deserialize(self, blob: Cord) -> DataPayload:
         """
         Deserializes a flat_tensor blob into a list of tensor metadata and tensors.
+
+        Note: deserialization does not preserve alignment information.
         """
 
         data = bytes(blob)
@@ -436,3 +439,14 @@ def deserialize(self, blob: Cord) -> DataPayload:
             payload.named_data[named_data.key] = entry
 
         return payload
+
+    def deserialize_to_named_data_store_output(
+        self, blob: bytes, name: str
+    ) -> NamedDataStoreOutput:
+        bytes = Cord(blob)
+        data_payload = self.deserialize(bytes)
+        return NamedDataStoreOutput(
+            buffers=data_payload.buffers,
+            pte_data={},
+            external_data={name:data_payload.named_data}
+        )