Returns human readable string of how much memory `num_bytes` fills.

def _num_bytes_to_human_readable(num_bytes):
  """Returns human readable string of how much memory `num_bytes` fills."""
  if num_bytes < (2 ** 10):
    return "%d B" % num_bytes
  elif num_bytes < (2 ** 20):
    return "%.3f KB" % (float(num_bytes) / (2 ** 10))
  elif num_bytes < (2 ** 30):
    return "%.3f MB" % (float(num_bytes) / (2 ** 20))
  else:
    return "%.3f GB" % (float(num_bytes) / (2 ** 30))