Add DontWant and DidntWant (the best names ever)
**Why?**
Because we want a way to say that we don't want a gradient for this primal. This will also allow us to skip work in transpose_fancy rules when the operand is a NullAccum.
**What's the API look like now?**
After you get the `f_vjp` from `jax.vjp`, you can call `with_refs` with these values:
`f_vjp.with_ref(x: Ref | DontWant | GradValue) : (GradRef | DidntWant | Jax.Array)`
Below is the mapping from user input -> internal accum -> output type.
* Ref (pytree) -- RefAccum -- GradRef (sentinal)
* DontWant (sentinal) -- NullAccum -- DidntWant (sentinal)
* GradValue (sentinal) -- ValAccum -- jax.Array (pytree)
Co-authored-by: Matthew Johnson <mattjj@google.com>
PiperOrigin-RevId: 879182894
