Return the Docker image to use for training.

        The :meth:`~sagemaker.estimator.EstimatorBase.fit` method, which does the model training,
        calls this method to find the image to use for model training.

        Returns:
            str: The URI of the Docker image.

def train_image(self):
        """Return the Docker image to use for training.

        The :meth:`~sagemaker.estimator.EstimatorBase.fit` method, which does the model training,
        calls this method to find the image to use for model training.

        Returns:
            str: The URI of the Docker image.
        """
        if self.image_name:
            return self.image_name
        else:
            return fw_utils.create_image_uri(self.sagemaker_session.boto_region_name,
                                             self._image_framework(),
                                             self.train_instance_type,
                                             self._image_version(),
                                             py_version=PYTHON_VERSION)