code: def testNestedMessageDescriptor(self):
      field_name = "optional_nested_message"
      proto_type = unittest_pb2.TestAllTypes
      self.assertEqual(
          proto_type.NestedMessage.DESCRIPTOR,
          proto_type.DESCRIPTOR.fields_by_name[field_name].message_type,
      )
  
imports: import unittest
  from google.protobuf.internal import test_bad_identifiers_pb2
  from google.protobuf import unittest_custom_options_pb2
  from google.protobuf import unittest_import_pb2
  from google.protobuf import unittest_import_public_pb2
  from google.protobuf import unittest_mset_pb2
  from google.protobuf import unittest_pb2
  from google.protobuf import unittest_no_generic_services_pb2
  from google.protobuf import service
label_desc: Use image node_id 1 for calling the GeneratorTest obj's underlying member method code with example usage: obj.testNestedMessageDescriptor() without return types