docs: def gradient(self, y_true, y_pred):
          """Returns the gradient of the L1 loss with respect to each prediction.
  
          Example:
              >>> import starboost as sb
              >>> y_true = [0, 0, 1]
              >>> y_pred = [0.3, 0, 0.8]
              >>> sb.losses.L1Loss().gradient(y_true, y_pred)
              array([ 1.,  0., -1.])
          """
          return np.sign(np.subtract(y_pred, y_true))