[JAX] Update ann to use XLA based fallback ApproxTopK.
[JAX] Update ann to use XLA based fallback ApproxTopK.

Other small changes:
* Restricts the operand type to float.
* Add more format annotations to the docstring.

PiperOrigin-RevId: 434749705

google/jax,google/jax,google/jax,google/jax