Add MLX dispatch for IfElse

[jessegrabowski]

There's no support for conditional logic in MLX, so the best we can do is mx.where. It ends up being similar to the limitations in jax -- jax.lax.cond gets rewritten to jax.lax.select, which is also where jnp.where ends up if you pass all 3 inputs (which we do in this case).

So IfElse sucks again -- it will evaluate both branches. But I submit that it's better than nothing.

[ricardoV94]

have an xpass strict

[jessegrabowski]

I also snuck a dispatch for TypeCastingOp (it's an identity function) so I split it into a separate commit