Data collator for token classification pads labels column when receives pytorch tensors
Author: markovalexander
Pull request: yes

# What does this PR do?

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Fixes DataCollatorForTokenClassification fails when trying to collate examples with "labels" column of pytorch tensors. 

I faced this issue yesterday and checked that it can be solved easily, so I did not open any Issue on github. 

## Before submitting
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Models:

- albert, bert, xlm: @LysandreJik
- blenderbot, bart, marian, pegasus, encoderdecoder,  t5: @patrickvonplaten, @patil-suraj
- longformer, reformer, transfoxl, xlnet: @patrickvonplaten
- fsmt: @stas00
- funnel: @sgugger
- gpt2: @patrickvonplaten, @LysandreJik
- rag: @patrickvonplaten, @lhoestq
- tensorflow: @LysandreJik

Library:

- benchmarks: @patrickvonplaten
- deepspeed: @stas00
- ray/raytune: @richardliaw, @amogkam
- text generation: @patrickvonplaten
- tokenizers: @n1t0, @LysandreJik
- trainer: @sgugger
- pipelines: @LysandreJik

Documentation: @sgugger

HF projects:

- datasets: [different repo](https://github.com/huggingface/datasets)
- rust tokenizers: [different repo](https://github.com/huggingface/tokenizers)

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- research_projects/bert-loses-patience: @JetRunner
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Comments:
The docs for this PR live [here](https://moon-ci-docs.huggingface.co/docs/transformers/pr_20244). All of your documentation changes will be reflected on that endpoint.

@sgugger

The docs for this PR live [here](https://moon-ci-docs.huggingface.co/docs/transformers/pr_20244). All of your documentation changes will be reflected on that endpoint.