def __init__(self, gamma=2.0, alpha=0.25):
    """Constructor.

    Args:
      gamma: exponent of the modulating factor (1 - p_t) ^ gamma.
      alpha: optional alpha weighting factor to balance positives vs negatives.
    """
    super(SigmoidFocalClassificationLoss, self).__init__()
    self._alpha = alpha
    self._gamma = gamma
Constructor.

    Args:
      gamma: exponent of the modulating factor (1 - p_t) ^ gamma.
      alpha: optional alpha weighting factor to balance positives vs negatives.
    
inference_api/src/main/object_detection/core/losses.py