message: Doc: add example for batch_flatten()
diff: @@ -2635,6 +2635,17 @@ def batch_flatten(x):
  Returns:
  A tensor.
  +
  + Examples:
  + Flatteinng a 3D tensor to 2D by collapsing the last dimension.
  +
  + ```python
  + >>> from tensorflow.keras import backend as K
  + >>> x_batch = K.ones(shape=(2, 3, 4, 5))
  + >>> x_batch_flatten = K.batch_flatten(x_batch)
  + >>> K.int_shape(x_batch_flatten)
  + (2, 60)
  + ```
  """
  x = array_ops.reshape(x, array_ops.stack([-1, prod(shape(x)[1:])]))
  return x
  