'Constructor.
Args:
is_training: A boolean indicating whether the training version of the
computation graph should be constructed.
first_stage_features_stride: Output stride of extracted RPN feature map.
reuse_weights: Whether to reuse variables. Default is None.
weight_decay: float weight decay for feature extractor (default: 0.0).'

def __init__(self, is_training, first_stage_features_stride, reuse_weights=None, weight_decay=0.0):

    self._is_training = is_training
    self._first_stage_features_stride = first_stage_features_stride
    self._reuse_weights = reuse_weights
    self._weight_decay = weight_decay
