Adding pipeline task aliases.
# What does this PR do?

Two tasks were sort of not aligned with the pipeline names.

`sentiment-analysis` -> `TextClassificationPipeline`
`ner` -> `TokenClassificationPipeline`

In order to make this change backward compatible, yet make the code more consistent, this PR 
introduces a TASK_ALIASES dictionary, which remaps a task name to another *canon* task name.

Previously working code should still be working, simply we are adding `text-classification` and `token-classification` tasks
available to `pipeline(...)` function.

## 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?
- [ ] 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.
- [ ] 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).
- [ ] Did you write any new necessary tests?


## Who can review?
@LysandreJik 
@philschmid 

HF projects:

- datasets: [different repo](https://github.com/huggingface/datasets)
- rust tokenizers: [different repo](https://github.com/huggingface/tokenizers)

Examples:

- maintained examples (not research project or legacy): @sgugger, @patil-suraj
- research_projects/bert-loses-patience: @JetRunner
- research_projects/distillation: @VictorSanh

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