Previously jax.core.DropVar was a singleton value (jax.core.dropvar) whose type was always jax.core.AbstractUnit. However, this type is misleading: a DropVar is an equation output, and typically we would expect it to have an array type. In particular, the unit type confuses new-style translation rules that expect to use the output aval on an equation as part of the lowering logic.
Instead, change DropVar to be a non-singleton subclass of Var instead with a flexible choice of aval.
PiperOrigin-RevId: 404071001