feat: add online data mixing plugin #718

Job	Run time
sample-config	12s
lint (attention-and-distributed-packing)	4m 10s
lint (framework)	4m 11s
lint (accelerated-peft)	4m 11s
lint (online-data-mixing)	3m 57s
lint (fused-ops-and-kernels)	4m 11s
lint (accelerated-moe)	4m 11s
	25m 3s

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