Fixed DataCollatorForLanguageModeling not accepting lists of lists
As discussed on Slack, currently `DataCollatorForLanguageModeling` and `DataCollatorForPermutationLanguageModeling` cannot take in lists of lists, as opposed to `default_data_collator`. This fixes this issue by calling `torch.Tensor` beforehand if a list of lists is detected.
# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/6685?src=pr&el=h1) Report
> Merging [#6685](https://codecov.io/gh/huggingface/transformers/pull/6685?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/1a779ad7ecb9e5215b6bd1cfa0153469d37e4274?el=desc) will **decrease** coverage by `0.00%`.
> The diff coverage is `75.00%`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/6685/graphs/tree.svg?width=650&height=150&src=pr&token=9qOlN6Hb1c)](https://codecov.io/gh/huggingface/transformers/pull/6685?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master    #6685      +/-   ##
==========================================
- Coverage   79.65%   79.64%   -0.01%     
==========================================
  Files         156      156              
  Lines       28250    28254       +4     
==========================================
+ Hits        22503    22504       +1     
- Misses       5747     5750       +3     
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/6685?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [src/transformers/data/data\_collator.py](https://codecov.io/gh/huggingface/transformers/pull/6685/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9kYXRhL2RhdGFfY29sbGF0b3IucHk=) | `89.70% <75.00%> (-1.21%)` | :arrow_down: |
| [src/transformers/generation\_tf\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/6685/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9nZW5lcmF0aW9uX3RmX3V0aWxzLnB5) | `86.46% <0.00%> (-0.26%)` | :arrow_down: |

------

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> Powered by [Codecov](https://codecov.io/gh/huggingface/transformers/pull/6685?src=pr&el=footer). Last update [1a779ad...dd1b689](https://codecov.io/gh/huggingface/transformers/pull/6685?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
