Fixed DataCollatorForLanguageModeling not accepting lists of lists (#6685)
Fixed DataCollatorForLanguageModeling not accepting lists of lists (#6685)

* Fixed DataCollatorForLanguageModeling + PermutationLanguageModeling not accepting lists of lists

* Update data_collator.py

* black was grumpy
huggingface/transformers,huggingface/transformers,huggingface/pytorch-transformers,huggingface/transformers,huggingface/transformers