'Extracts first stage RPN features.
This function is responsible for extracting feature maps from preprocessed
images.  These features are used by the region proposal network (RPN) to
predict proposals.
Args:
preprocessed_inputs: A [batch, height, width, channels] float tensor
representing a batch of images.
scope: A scope name.
Returns:
rpn_feature_map: A tensor with shape [batch, height, width, depth]'

def extract_proposal_features(self, preprocessed_inputs, scope):

    with tf.variable_scope(scope, values=[preprocessed_inputs]):
        return self._extract_proposal_features(preprocessed_inputs, scope)
