message: added example for flat_map
diff: @@ -1007,8 +1007,20 @@ class Dataset(object):
  return ParallelMapDataset(self, map_func, num_parallel_calls)
  def flat_map(self, map_func):
  - """Maps `map_func` across this dataset and flattens the result. Will produce identical results to 'tf.data.Dataset.interleave'
  + """Maps `map_func` across this dataset and flattens the result.
  + Will produce similar results to `tf.data.Dataset.interleave(cycle_length=1)`.
  + Use `flat_map` if you want to make sure, that the order of your dataset stays the same.
  + For example:
  +
  + ```python
  + # NOTE: The following examples use `{ ... }` to represent the
  + # contents of a dataset. '[...]' represents a tensor.
  + a = {[1,2,3,4,5], [6,7,8,9], [10]}
  +
  + a.flat_map(lambda x: Dataset.from_tensors(x)) ==
  + {[1,2,3,4,5,6,7,8,9,10]}
  + ```
  Args:
  map_func: A function mapping a nested structure of tensors (having shapes
  and types defined by `self.output_shapes` and `self.output_types`) to a
  