message: Add info about Dataset shard's deterministic nature
diff: @@ -973,6 +973,23 @@ class DatasetV2(tracking_base.Trackable, composite_tensor.CompositeTensor):
  def shard(self, num_shards, index):
  """Creates a `Dataset` that includes only 1/`num_shards` of this dataset.
  + For example:
  +
  + ```python
  + # Create a Dataset with 60 elements.
  + A = tf.data.Dataset.range(60) # ==> [0, 1, 2, 3, ..., 57, 58, 59]
  +
  + # Create 3 Datasets, each with 20 elements from Dataset A.
  + B = A.shard(num_shards=3, index=0) # ==> [0, 3, 6, 9, ..., 51, 54, 57]
  + C = A.shard(num_shards=3, index=1) # ==> [1, 4, 7, 10, ..., 52, 55, 58]
  + D = A.shard(num_shards=3, index=2) # ==> [2, 5, 8, 11, ..., 53, 56, 59]
  +
  + # There is no overlap between Datasets B, C and D.
  + ```
  +
  + `shard` is a deterministic operator; the returned Dataset depends only
  + on the target Dataset, and the values of `num_shards` and `index`.
  +
  This dataset operator is very useful when running distributed training, as
  it allows each worker to read a unique subset.
  