SaveAt dense and fn

When using dense=True in SaveAt and a custom fn, I was expected that the interpolation of the sol will respect the custom fn but it's not the case.

For now, it seems impossible to have an interpolation only on "statisctics" which have be saved with a custom fn without saved all the info.

    def f(t, x, args):
        y = x
        return y

    solver = diffrax.Kvaerno5()
    stepsize_controller = diffrax.PIDController(rtol=1e-8, atol=1e-8)  # rtol=1e-8, atol=1e-8


    t0 = 0.
    t1 = 10
    y0 = jnp.array([3.0,2.0,10.0])
    dt0 = 0.0002

    sol = diffrax.diffeqsolve(
        diffrax.ODETerm(f),
        solver,
        t0,
        t1,
        dt0,
        y0,
        saveat=diffrax.SaveAt(dense=True,steps=True,fn=lambda t,y,args: y[:2]),
        stepsize_controller=stepsize_controller,
        max_steps=100000,
    )

    print(sol.interpolation.infos) # dim 3 ko
    print(sol.ys) # -> dim 2 ok

Start by reproducing the provided diffrax.diffeqsolve example with SaveAt(dense=True, steps=True, fn=...) and compare sol.interpolation.infos with sol.ys. Trace how SaveAt's custom fn is applied to saved values versus dense interpolation; done means dense interpolation respects the custom function without retaining the full state.