code: def loss_gen(self, gen, y_fake):
      batchsize = len(y_fake)
      loss = F.sum(F.softplus(-y_fake)) / batchsize
      chainer.report({"loss": loss}, gen)
      return loss
  
file_path: chainer/examples/chainermn/dcgan/updater.py
imports: from __future__ import print_function
  import chainer
  import chainer.functions
  from chainer import Variable
label_desc: Use image node_id 3 for calling the DCGANUpdater obj's underlying member method code with example usage: obj.loss_gen(gen, y_fake) and returns: loss