Allow PaddingFree to work with DataCollatorForCompletionOnlyLM #320

Job	Run time
sample-config	7s
lint (framework)	3m 40s
lint (accelerated-peft)	3m 35s
lint (fused-ops-and-kernels)	3m 30s
lint (attention-and-distributed-packing)	4m 9s
	15m 1s

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