docstr: Mask L1 Loss
docstr_tok: ["Mask","L1","Loss"]
code: def forward(self, pred, gt, mask):
      """
      Mask L1 Loss
      """
      loss = (paddle.abs(pred - gt) * mask).sum() / (
          mask.sum() + self.eps
      )
      loss = paddle.mean(loss)
      return loss
  
imports: from __future__ import absolute_import
  from __future__ import division
  from __future__ import print_function
  import numpy
  import paddle
  from paddle import nn
  import paddle.nn.functional
label_desc: Use image node_id 2 for calling the MaskL1Loss obj's underlying member method code with example usage: obj.forward(pred, gt, mask) and returns: loss