code: def forward(ctx, dists, pix_to_face, sigma):
      alphas = _C.sigmoid_alpha_blend(dists, pix_to_face, sigma)
      ctx.save_for_backward(dists, pix_to_face, alphas)
      ctx.sigma = sigma
      return alphas
  
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 1 for calling the _SigmoidAlphaBlend obj's underlying member method code with example usage: obj.forward(ctx, dists, pix_to_face, sigma) and returns: alphas