def _compute_loss(self, prediction_tensor, target_tensor, weights):
    """Compute loss function.

    Args:
      prediction_tensor: A float tensor of shape [batch_size, num_anchors,
        num_classes] representing the predicted logits for each class
      target_tensor: A float tensor of shape [batch_size, num_anchors,
        num_classes] representing logit classification targets
      weights: a float tensor of shape, either [batch_size, num_anchors,
        num_classes] or [batch_size, num_anchors, 1]. If the shape is
        [batch_size, num_anchors, 1], all the classses are equally weighted.

    Returns:
      loss: a float tensor of shape [batch_size, num_anchors]
        representing the value of the loss function.
    """
    weights = tf.reduce_mean(weights, axis=2)
    num_classes = prediction_tensor.get_shape().as_list()[-1]
    target_tensor = self._scale_and_softmax_logits(target_tensor)
    prediction_tensor = tf.divide(prediction_tensor, self._logit_scale,
                                  name='scale_logits')

    per_row_cross_ent = (tf.nn.softmax_cross_entropy_with_logits(
        labels=tf.reshape(target_tensor, [-1, num_classes]),
        logits=tf.reshape(prediction_tensor, [-1, num_classes])))
    return tf.reshape(per_row_cross_ent, tf.shape(weights)) * weights
Compute loss function.

    Args:
      prediction_tensor: A float tensor of shape [batch_size, num_anchors,
        num_classes] representing the predicted logits for each class
      target_tensor: A float tensor of shape [batch_size, num_anchors,
        num_classes] representing logit classification targets
      weights: a float tensor of shape, either [batch_size, num_anchors,
        num_classes] or [batch_size, num_anchors, 1]. If the shape is
        [batch_size, num_anchors, 1], all the classses are equally weighted.

    Returns:
      loss: a float tensor of shape [batch_size, num_anchors]
        representing the value of the loss function.
    
inference_api/src/main/object_detection/core/losses.py