Training with a custom service account needs a new field named service account
trainingInput:
  serviceAccount: CUSTOM_SERVICE_ACCOUNT

https://cloud.google.com/ai-platform/training/docs/custom-service-account


airflow/providers/google/cloud/operators/mlengine.py
hook = MLEngineHook(
            gcp_conn_id=self._gcp_conn_id,
            delegate_to=self._delegate_to,
            impersonation_chain=self._impersonation_chain,
        )
        if self._operation == 'create':
            return hook.create_model(project_id=self._project_id, model=self._model)
        elif self._operation == 'get':
            return hook.get_model(project_id=self._project_id, model_name=self._model['name'])
        else:
            raise ValueError('Unknown operation: {}'.format(self._operation))
class MLEngineCreateModelOperator(BaseOperator):