message: Update the optimizer documentation to explicitly note that `_resource_apply_sparse` expects the gradients to be a `tf.IndexedSlices`.
diff: @@ -296,8 +296,11 @@ class OptimizerV2(trackable.Trackable):
  If you intend to create your own optimization algorithm, simply inherit from
  this class and override the following methods:
  - - `_resource_apply_dense` (update variable given gradient tensor is dense)
  - - `_resource_apply_sparse` (update variable given gradient tensor is sparse)
  + - `_resource_apply_dense` (update variable given gradient tensor is a dense
  + `tf.Tensor`)
  + - `_resource_apply_sparse` (update variable given gradient tensor is a
  + sparse `tf.IndexedSlices`. The most common way for this to happen
  + is if you are taking the gradient through a `tf.gather`.)
  - `_create_slots`
  (if your optimizer algorithm requires additional variables)
  - `get_config`
  