'Returns the default target for anchors to regress to.
Default regression targets are set to zero (though in
this implementation what these targets are set to should
not matter as the regression weight of any box set to
regress to the default target is zero).
Returns:
default_target: a float32 tensor with shape [1, box_code_dimension]'

def _default_regression_target(self):

    return tf.constant([(self._box_coder.code_size * [0])], tf.float32)
