class_docstr: KNN (CUDA) based on heap data structure.
  Modified from `PAConv <https://github.com/CVMI-Lab/PAConv/tree/main/
  scene_seg/lib/pointops/src/knnquery_heap>`_.
  
  Find k-nearest points.
code: def backward(ctx, a=None):
      return None, None, None
  
imports: import torch
  from torch.autograd import Function
  from ..utils import ext_loader
label_desc: Use image node_id 2 for calling the KNN obj's underlying member method code with example usage: obj.backward(ctx, a) and returns: None,  None,  None