code: def batch_reshape_mask_left(
      input_t, selected, pad_idx=0, left_align_mask=None
  ):
      """
      Left-aligns all ``selected" values in input_t, which is a batch of examples.
          - input_t: >=2D tensor (N, M, *)
          - selected: 2D torch.Bool tensor, 2 dims same size as first 2 dims of `input_t` (N, M)
          - pad_idx represents the padding to be used in the output
          - left_align_mask: if already precomputed, pass the alignment mask in
              (mask on the output, corresponding to `selected` on the input)
      Example:
          input_t  = [[1,2,3,4],[5,6,7,8]]
          selected = [[0,1,0,1],[1,1,0,1]]
          output   = [[2,4,0],[5,6,8]]
      """
      batch_num_selected = selected.sum(1)
      max_num_selected = batch_num_selected.max()
  
      # (bsz, 2)
      repeat_freqs = torch.stack(
          [batch_num_selected, max_num_selected - batch_num_selected],
          dim=-1,
      )
      # (bsz x 2,)
      repeat_freqs = repeat_freqs.view(-1)
  
      if left_align_mask is None:
          # (bsz, 2)
          left_align_mask = (
              torch.zeros(input_t.size(0), 2).to(input_t.device).bool()
          )
          left_align_mask[:, 0] = 1
          # (bsz x 2,): [1,0,1,0,...]
          left_align_mask = left_align_mask.view(-1)
          # (bsz x max_num_selected,): [1 xrepeat_freqs[0],0 x(M-repeat_freqs[0]),1 xrepeat_freqs[1],0 x(M-repeat_freqs[1]),...]
          left_align_mask = left_align_mask.repeat_interleave(
              repeat_freqs
          )
          # (bsz, max_num_selected)
          left_align_mask = left_align_mask.view(-1, max_num_selected)
  
      # reshape to (bsz, max_num_selected, *)
      input_reshape = (
          torch.Tensor(left_align_mask.size() + input_t.size()[2:])
          .to(input_t.device, input_t.dtype)
          .fill_(pad_idx)
      )
      input_reshape[left_align_mask] = input_t[selected]
      # (bsz, max_num_selected, *); (bsz, max_num_selected)
      return input_reshape, left_align_mask
  
label_desc: Use image node_id 1 for calling a global function with example usage: batch_reshape_mask_left(input_t, selected, pad_idx, left_align_mask) and returns: input_reshape,  left_align_mask