experiment - use first taskrun as anchor pod

[QuanZhang-William]

This is an experiment to achieve node affinity without using placeholder pod as we do today in affinity assistant. In this experiment, the first PipelienTask pod is used as the "anchor" pod (which can be scheduled to any node). Pod affinity terms are only applied follow-up pods, so that the pods can be anchored to the first PipelineTask pod.

The main issue with this idea is that if the anchor pod finishes before the followup pods are started, it runs into scheduling conflict as there is no node meets the affinity requirement (inter pod affinity only considers running pods).

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