Checkpointable: Have RNN wrappers add their cells as dependencies

Checkpointable: Have RNN wrappers add their cells as dependencies

Also marks _SlimRNNCell as not checkpointable, and adds a more convenient way to tag such classes.

Ideally adding a wrapper around a cell wouldn't break a checkpoint. This could look like RNN cell wrappers inheriting the dependencies of the cell they're wrapping. Possible to add that later if there's demand, or users can just add a dependency on wrapper._cell in addition to/instead of the wrapper when modifying programs.

Fixes #19208.

PiperOrigin-RevId: 196202366