message: Add an example of an `IndexedSlices`
diff: @@ -81,6 +81,18 @@ class IndexedSlices(internal.NativeObject, composite_tensor.CompositeTensor):
  gradients for operations that have sparse gradients
  (e.g. `tf.gather`).
  + >>> v = tf.Variable([[0.,1, 2], [2, 3, 4], [4, 5, 6], [6, 7, 8]])
  + >>> with tf.GradientTape() as tape:
  + ... r = tf.gather(v, [1,3])
  + >>> index_slices = tape.gradient(r,v)
  + >>> index_slices
  + <...IndexedSlices object ...>
  + >>> index_slices.indices.numpy()
  + array([1, 3], dtype=int32)
  + >>> index_slices.values.numpy()
  + array([[1., 1., 1.],
  + [1., 1., 1.]], dtype=float32)
  +
  Contrast this representation with
  `tf.sparse.SparseTensor`,
  which uses multi-dimensional indices and scalar values.
  