class_docstr: Find the top-3 nearest neighbors of the target set from the source set.
  
  Please refer to `Paper of PointNet++ <https://arxiv.org/abs/1706.02413>`_
  for more details.
code: def backward(ctx, a=None, b=None):
      return None, None
  
file_path: lavis/lavis/common/annotator/uniformer/mmcv/ops/three_nn.py
imports: from typing import Tuple
  import torch
  from torch.autograd import Function
  from ..utils import ext_loader
label_desc: Use image node_id 2 for calling the ThreeNN obj's underlying member method code with example usage: obj.backward(ctx, a, b) and returns: None,  None