message: Update grouping.py
  Add example for bucket_by_sequence_length.
diff: @@ -167,6 +167,56 @@ def bucket_by_sequence_length(element_length_func,
  [[ 0 0]
  [21 22]]
  + There is also a possibility to pad the dataset till the bucket boundary.
  + You can also provide which value to be used while padding the data.
  + Below example uses `-1` as padding and it also shows the input data being bucketizied to two buckets
  + "[0,3], [4,6]"
  +
  + >>> elements = [
  + ... [0], [1, 2, 3, 4], [5, 6, 7],
  + ... [7, 8, 9, 10, 11], [13, 14, 15, 16, 19, 20], [21, 22]]
  +
  + >>> dataset = tf.data.Dataset.from_generator(
  + ... lambda: elements, tf.int32, output_shapes=[None])
  +
  + >>> dataset = dataset.apply(
  + ... tf.data.experimental.bucket_by_sequence_length(
  + ... element_length_func=lambda elem: tf.shape(elem)[0],
  + ... bucket_boundaries=[4, 7],
  + ... bucket_batch_sizes=[2, 2, 2],
  + ... pad_to_bucket_boundary=True,
  + ... padding_values=-1))
  +
  + >>> for elem in dataset.as_numpy_iterator():
  + ... print(elem)
  + [[ 0 -1 -1]
  + [ 5 6 7]]
  + [[ 1 2 3 4 -1 -1]
  + [ 7 8 9 10 11 -1]]
  + [[21 22 -1]]
  + [[13 14 15 16 19 20]]
  +
  + When using `pad_to_bucket_boundary` option, it can be seen that it is not always possible
  + to maintain the bucket batch size.
  + You can drop the batches that do not maintain the bucket batch size by using the option `drop_remainder`.
  + Using the same input data as in the above example you get the following result.
  +
  + >>> dataset = dataset.apply(
  + ... tf.data.experimental.bucket_by_sequence_length(
  + ... element_length_func=lambda elem: tf.shape(elem)[0],
  + ... bucket_boundaries=[4, 7],
  + ... bucket_batch_sizes=[2, 2, 2],
  + ... pad_to_bucket_boundary=True,
  + ... padding_values=-1,
  + ... drop_remainder=True))
  +
  + >>> for elem in dataset.as_numpy_iterator():
  + ... print(elem)
  + [[ 0 -1 -1]
  + [ 5 6 7]]
  + [[ 1 2 3 4 -1 -1]
  + [ 7 8 9 10 11 -1]]
  +
  Args:
  element_length_func: function from element in `Dataset` to `tf.int32`,
  determines the length of the element, which will determine the bucket it
  