'Return list of spatial dimensions for each feature map in a list.
Args:
feature_maps: a list of tensors where the ith tensor has shape
[batch, height_i, width_i, depth_i].
Returns:
a list of pairs (height, width) for each feature map in feature_maps'

def _get_feature_map_spatial_dims(self, feature_maps):

    feature_map_shapes = [shape_utils.combined_static_and_dynamic_shape(feature_map) for feature_map in feature_maps]
    return [(shape[1], shape[2]) for shape in feature_map_shapes]
