Improve model adapter reconcile workflow stability

[Jeffwan]

Pull Request Description

1. Polish the workflow

I1001 10:27:10.704733   11452 modeladapter_controller.go:266] "Adding finalizer for ModelAdapter" ModelAdapter="aibrix-system/lora-1"
I1001 10:27:18.798096   11452 modeladapter_controller.go:452] "model adapter reconcile" Update CR status="lora-1" status={"phase":"Pending"}
I1001 10:27:33.971201   11452 leastadapters.go:53] "pod selected with least model adapters" pod="aibrix-system/llama2-70b-8fd6c849b-vd9rl"
I1001 10:27:39.941767   11452 modeladapter_controller.go:452] "model adapter reconcile" Update CR status="lora-1" status={"phase":"Scheduled","conditions":[{"type":"Initialized","status":"Unknown","lastTransitionTime":"2024-10-01T17:27:18Z","reason":"ModelAdapterPending","message":"Starting reconciliation"}],"instances":["llama2-70b-8fd6c849b-vd9rl"]}
I1001 10:28:01.208669   11452 modeladapter_controller.go:715] "Creating a new service" service="aibrix-system/lora-1"
I1001 10:28:13.041924   11452 modeladapter_controller.go:807] "Creating a new EndpointSlice" endpointslice="aibrix-system/lora-1"
I1001 10:28:44.503105   11452 modeladapter_controller.go:439] "model adapter reconcile" Update CR status="lora-1" status={"phase":"Running","conditions":[{"type":"Initialized","status":"Unknown","lastTransitionTime":"2024-10-01T17:27:18Z","reason":"ModelAdapterPending","message":"Starting reconciliation"},{"type":"Scheduled","status":"True","lastTransitionTime":"2024-10-01T17:27:39Z","reason":"Scheduled","message":"ModelAdapter aibrix-system/lora-1 has been allocated to pod aibrix-system/llama2-70b-8fd6c849b-vd9rl"}],"instances":["llama2-70b-8fd6c849b-vd9rl"]}

2. Update the conditions. the major change is not to use it as state machine, only report necessary failures.

Related Issues

Resolves: #136

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