title: How to use horovod for allreduce operations on normal values？
body: <p>Horovod can easily calculate the gradient of tensorflow. Can we use horovod to calculate ordinary values?For example:</p>
  
  <pre><code>import horovod.tensorflow as hvd
  import numpy as np
  hvd.init()
  hvd_r=int(hvd.rank())
  #each process compute a small part of something and then compute the average etc.
  test_array=np.random.rand(100,100,100)
  #compute a small part
  x=np.mean(test_array[hvd_r*10:(hvd_r+1)*(10),:,:])
  #compute the average for all processes
  y=hvd.sth(x)
  #only one process print out the result
  if(hvd_r==0):
      print("mean of the big array is %f"%y)
  </code></pre>