feat(scheduler): Decoupled Scheduler with Router and Adding Router Readiness Check

[christ-tt]

• Add a check that there's at least one ready pipeline, defined by all nodes in it are is_active, before dispatching requests.
• Decoupled router and scheduler; so we don't conditioning on strategy; and we let router handles the routing related tasks.

TODO

• Maybe rename node.is_active , as the active here means even after we have assigned layers / pipeline formation, node will need time for parameters loading, engine setup, cuda graph init, etc. The naming here may causing confusion for how we define active and standby for our nodes.