Add method to export the "final" metadata string in MetadataWriter

Extra fields could be filled by MetadataPopulator, such as min_parser_version. Added the method `get_populated_metadata_json` to return the final metadata string.

PiperOrigin-RevId: 406744227

tensorflow_lite_support/metadata/python/metadata_writers/metadata_writer.py

@@ -42,6 +42,7 @@ def __init__(self,
     self._model_buffer = model_buffer
     self._metadata_buffer = metadata_buffer
     self._associated_files = associated_files if associated_files else []
+    self._populated_model_buffer = None
 
   @classmethod
   def create_from_metadata_info(
@@ -156,19 +157,45 @@ def populate(self) -> bytearray:
     Returns:
       A new model buffer with the metadata and associated files.
     """
+    if self._populated_model_buffer:
+      return self._populated_model_buffer
+
     populator = _metadata.MetadataPopulator.with_model_buffer(
         self._model_buffer)
     if self._model_buffer is not None:
       populator.load_metadata_buffer(self._metadata_buffer)
     if self._associated_files:
       populator.load_associated_files(self._associated_files)
     populator.populate()
-    return populator.get_model_buffer()
+    self._populated_model_buffer = populator.get_model_buffer()
+    return self._populated_model_buffer
 
   def get_metadata_json(self) -> str:
-    """Gets the generated metadata string in JSON format."""
+    """Gets the generated JSON metadata string before populated into model.
+
+    This method returns the metadata buffer before populated into the model.
+    More fields could be filled by MetadataPopulator, such as
+    min_parser_version. Use get_populated_metadata_json() if you want to get the
+    final metadata string.
+
+    Returns:
+      The generated JSON metadata string before populated into model.
+    """
     return _metadata.convert_to_json(bytes(self._metadata_buffer))
 
+  def get_populated_metadata_json(self) -> str:
+    """Gets the generated JSON metadata string after populated into model.
+
+    More fields could be filled by MetadataPopulator, such as
+    min_parser_version. Use get_metadata_json() if you want to get the
+    original metadata string.
+
+    Returns:
+      The generated JSON metadata string after populated into model.
+    """
+    displayer = _metadata.MetadataDisplayer.with_model_buffer(self.populate())
+    return displayer.get_metadata_json()
+
 
 # If tensor name in metadata is empty, default to the tensor name saved in
 # the model.

tensorflow_lite_support/metadata/python/tests/metadata_writers/metadata_writer_test.py

@@ -156,6 +156,19 @@ def test_get_metadata_json_should_succeed(self):
     expected_json = test_utils.load_file(_EXPECTED_DUMMY_NO_VERSION_JSON, "r")
     self.assertEqual(metadata_json, expected_json)
 
+  def test_get_populated_metadata_json_should_succeed(self):
+    model_buffer = test_utils.load_file(_MODEL)
+    model_metadata, input_metadata, output_metadata = (
+        self._create_dummy_metadata())
+
+    writer = metadata_writer.MetadataWriter.create_from_metadata(
+        model_buffer, model_metadata, [input_metadata], [output_metadata],
+        [_LABEL_FILE])
+    metadata_json = writer.get_populated_metadata_json()
+
+    expected_json = test_utils.load_file(_EXPECTED_DUMMY_JSON, "r")
+    self.assertEqual(metadata_json, expected_json)
+
   def _assert_correct_metadata(self,
                                model_with_metadata,
                                expected_json_file,