code: def _flatten(self, sources, lengths):
      return paddle.concat([t[:l] for t, l in zip(sources, lengths)])
  
file_path: PaddleOCR/ppocr/losses/rec_vl_loss.py
imports: from __future__ import absolute_import
  from __future__ import division
  from __future__ import print_function
  import paddle
  from paddle import nn
label_desc: Use image node_id 3 for calling the VLLoss obj's underlying member method code with example usage: obj._flatten(sources, lengths) and returns: paddle