print counters hierarchically.

  Each counter is a pair of a string and a number.
  The string can have slashes, meaning that the number also counts towards
  each prefix.  e.g.  "parameters/trainable" counts towards both "parameters"
  and "parameters/trainable".

  Args:
    counters: a list of (string, number) pairs

  Returns:
    a string

def pretty_print_counters(counters):
  """print counters hierarchically.

  Each counter is a pair of a string and a number.
  The string can have slashes, meaning that the number also counts towards
  each prefix.  e.g.  "parameters/trainable" counts towards both "parameters"
  and "parameters/trainable".

  Args:
    counters: a list of (string, number) pairs

  Returns:
    a string
  """
  totals = collections.defaultdict(int)
  for (name, val) in counters:
    prefixes = [name[:i] for i in xrange(len(name)) if name[i] == "/"] + [name]
    for p in prefixes:
      totals[p] += val
  parts = []
  for name, val in sorted(six.iteritems(totals)):
    parts.append(" " * name.count("/") + "%s: %.3g" % (name, val))
  return "\n".join(parts)