Add batching in TokenClassificationPipeline
# What does this PR do?

Currently, the NER pipeline in transformers iterates through the list of input sentences and processes them, sequentially.
This PR adds batching support in the pipeline to decrease latency and use GPU more efficiently.

Relevant Issue :-  #11244 

## Benchmark Report

### Without Batching (CPU)
Device: CPU
No. examples: 1000
Time taken: 283.27826976776123

Device: GPU
No. examples: 1000
Time taken: 17.89318561553955

Please check the benchmark gist [here](https://gist.github.com/parakalan/88b613ed4ca0001afb60448996f6b62a)

### Without Batching (CPU)
Device: CPU
No. examples: 1000
Batch Size: 512
Time taken: 121.81582999229431

Device: GPU
No. examples: 1000
Batch Size: 512
Time taken: 2.780881404876709

Please check the benchmark gist [here](https://gist.github.com/parakalan/f1fa25f25b8a70125145afbcbbeac85f)

## Before submitting
- [ ] This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
- [x] Did you read the [contributor guideline](https://github.com/huggingface/transformers/blob/master/CONTRIBUTING.md#start-contributing-pull-requests),
      Pull Request section?
- [x] Was this discussed/approved via a Github issue or the [forum](https://discuss.huggingface.co/)? Please add a link
      to it if that's the case. - https://github.com/huggingface/transformers/issues/11244
- [x] Did you make sure to update the documentation with your changes? Here are the
      [documentation guidelines](https://github.com/huggingface/transformers/tree/master/docs), and
      [here are tips on formatting docstrings](https://github.com/huggingface/transformers/tree/master/docs#writing-source-documentation).
- [x] Did you write any new necessary tests?

FYI there is also work done on this pipeline in https://github.com/huggingface/transformers/pull/10568 if you want to give it a look! It doesn't concern batching, however.<|||||>Thanks, let me check that out. <|||||>Please review this  @LysandreJik , @Narsil , @joshdevins<|||||>Closing this PR based on @Narsil's review. Thanks