code: def Allreduce2(net, blobs, reduced_affix, gpu_indices):
      """Allreduce for 2 gpus.
  
      Algorithm: 0r <- 0 + 1, 1r <- 0r, where r means "reduced"
      """
      a, b = blobs
      gpu_a, gpu_b = gpu_indices
      a_reduced = net.Add(
          [a, b], a + reduced_affix, device_option=OnGPU(gpu_a)
      )
      b_reduced = a_reduced.Copy(
          [], b + reduced_affix, device_option=OnGPU(gpu_b)
      )
      return a_reduced, b_reduced
  
file_path: pytorch/caffe2/python/muji.py
imports: import numpy
  from caffe2.proto import caffe2_pb2
  from caffe2.python import workspace
label_desc: Use image node_id 4 for calling a global function with example usage: Allreduce2(net, blobs, reduced_affix, gpu_indices) and returns: a_reduced,  b_reduced