**Description**

The operator is arguably unnecessarily limited to AI Platform’s standard images. The only change that is required to lift this constraint is making `package_uris` and `training_python_module` optional with default values `[]` and `None`, respectively. Then, using `master_config`, one can supply `imageUri` and run any image of choice.

**Use case / motivation**

This will open up for running arbitrary images on AI Platform.

**Are you willing to submit a PR?**

If the above sounds reasonable, I can open pull requests.
tests/providers/google/cloud/operators/test_mlengine.py
)
        hook_instance.create_job.assert_called_once_with(
            project_id='test-project',
            job={'jobId': 'test_prediction', 'predictionInput': input_with_version},
            use_existing_job_fn=ANY,
        )
        self.assertEqual(success_message['predictionOutput'], prediction_output)
    @patch('airflow.providers.google.cloud.operators.mlengine.MLEngineHook')
    def test_success_with_uri(self, mock_hook):
        input_with_uri = self.INPUT_MISSING_ORIGIN.copy()
        input_with_uri['uri'] = 'gs://my_bucket/my_models/savedModel'
        success_message = self.SUCCESS_MESSAGE_MISSING_INPUT.copy()
        success_message['predictionInput'] = input_with_uri
        hook_instance = mock_hook.return_value
        hook_instance.get_job.side_effect = HttpError(
            resp=httplib2.Response({'status': 404}), content=b'some bytes'
        )
        hook_instance.create_job.return_value = success_message
        prediction_task = MLEngineStartBatchPredictionJobOperator(
            job_id='test_prediction',
            project_id='test-project',
            region=input_with_uri['region'],
            data_format=input_with_uri['dataFormat'],
            input_paths=input_with_uri['inputPaths'],
            output_path=input_with_uri['outputPath'],
            uri=input_with_uri['uri'],
            dag=self.dag,
            task_id='test-prediction',
        )
        prediction_output = prediction_task.execute(None)