code: def backward(ctx, grad_alphas):
      dists, pix_to_face, alphas = ctx.saved_tensors
      sigma = ctx.sigma
      grad_dists = _C.sigmoid_alpha_blend_backward(
          grad_alphas, alphas, dists, pix_to_face, sigma
      )
      return grad_dists, None, None
  
file_path: pytorch3d/pytorch3d/renderer/blending.py
imports: from typing import NamedTuple, Sequence, Union
  import torch
  from pytorch3d import _C
  from pytorch3d.common.datatypes import Device
label_desc: Use image node_id 2 for calling the _SigmoidAlphaBlend obj's underlying member method code with example usage: obj.backward(ctx, grad_alphas) and returns: grad_dists,  None,  None