'To be overridden by implementations.
Args:
feature_map_shape_list: list of (height, width) pairs in the format
[(height_0, width_0), (height_1, width_1), ...] that the generated
anchors must align with.
**params: parameters for anchor generation op
Returns:
boxes: a BoxList holding a collection of N anchor boxes'

@abstractmethod
def _generate(self, feature_map_shape_list, **params):

    pass
