message: Use enumerate instead of manually incrementing i in
  multi_device_iterator_ops.py
  Using enumerate(..) both shortens the code, and is more pythonic due to
  reducing the chance a future refactoring accidentally misses the += 1.
diff: @@ -201,8 +201,7 @@ class MultiDeviceIterator(object):
  # into the device side from its input. It might be useful in rewriting.
  # Create the per device iterators.
  self._device_iterators = []
  - i = 0
  - for device in self._devices:
  + for i, device in enumerate(self._devices):
  ds = _PerDeviceGenerator(
  i, self._multi_device_iterator_resource, self._incarnation_id,
  self._source_device_tensor, device, self._dataset.output_shapes,
  @@ -211,7 +210,6 @@ class MultiDeviceIterator(object):
  ds = ds.prefetch(prefetch_buffer_size)
  with ops.device(device):
  self._device_iterators.append(ds.make_initializable_iterator())
  - i += 1
  device_iterator_initializers = [
  iterator.initializer for iterator in self._device_iterators
  @@ -220,21 +218,17 @@ class MultiDeviceIterator(object):
  def get_next(self):
  result = []
  - i = 0
  - for device in self._devices:
  + for i, device in enumerate(self._devices):
  with ops.device(device):
  result.append(self._device_iterators[i].get_next())
  - i += 1
  return result
  def get_next_as_optional(self):
  result = []
  - i = 0
  - for device in self._devices:
  + for i, device in enumerate(self._devices):
  with ops.device(device):
  result.append(iterator_ops.get_next_as_optional(
  self._device_iterators[i]))
  - i += 1
  return result
  @property
  