[pjrt] NFC: Always use concrete async value in PjRtFuture<T>
[pjrt] NFC: Always use concrete async value in PjRtFuture<T>

We plan to make absl::StatusOr<T> an implicit payload of PjRtFuture<T> and for consistency always use AsyncValueRef with concrete payload instead of relying on AsyncValue error semantics.

PjRtFuture can't be constructed from async values passed from a user (only from a promise) so we can safely ignore the error bit as we never use it.

This is a first CL in preparation for making absl::StatusOr implicit in PjRtFuture<T>.
