Add support for non-rust implemented tokenization for `__getitem__` m…
…ethod.

# What does this PR do?

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Fixes # (issue)


## Before submitting
- [ ] This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
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Models:

- text models: @ArthurZucker and @younesbelkada
- vision models: @amyeroberts
- speech models: @sanchit-gandhi
- graph models: @clefourrier

Library:

- flax: @sanchit-gandhi
- generate: @gante
- pipelines: @Narsil
- tensorflow: @gante and @Rocketknight1
- tokenizers: @ArthurZucker
- trainer: @sgugger

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- deepspeed: HF Trainer: @stas00, Accelerate: @pacman100
- ray/raytune: @richardliaw, @amogkam

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

Maintained examples (not research project or legacy):

- Flax: @sanchit-gandhi
- PyTorch: @sgugger
- TensorFlow: @Rocketknight1

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The docs for this PR live [here](https://moon-ci-docs.huggingface.co/docs/transformers/pr_23645). All of your documentation changes will be reflected on that endpoint.<|||||>Hi @jacklanda, thanks for opening this PR! 

Could you add some more detail to the PR description about the issue this is addressing or feature this is adding? 

cc @younesbelkada @ArthurZucker <|||||>> Hi @jacklanda, thanks for opening this PR!
> 
> Could you add some more detail to the PR description about the issue this is addressing or feature this is adding?
> 
> cc @younesbelkada @ArthurZucker

Hi @amyeroberts , this PR is going to add a support for the usage scenario of "getting a slice from the batch-tokenized sequences". 

Without this PR, it seems to raise `KeyError` with the following message `KeyError: 'Indexing with integers (to access backend Encoding for a given batch index) is not available when using Python based tokenizers'`

P.S. The above scenario could be reproduced by using some models new uploaded but not support to Rust-implemented tokenization, such as `fnlp/moss-moon-003-sft`. Also we can run a examplar script for reproducing this issue:

```python3
from transformers import AutoTokenizer

tok = AutoTokenizer.from_pretrained("fnlp/moss-moon-003-sft", trust_remote_code=True)
tok.add_special_tokens({"pad_token": "[PAD]"})

texts = ["Today is a good day!", "It's a good idea!", "How's going?"]
batch_tok = tok(texts, padding=True)
print(batch_tok[0:3])  # report `KeyError` here
```

All in all, I think it seems useful to implement `__getitem__` method behind it in Python side :)<|||||>Why this auto pipeline always failed? <|||||>@jacklanda By 'auto pipeline' are you referring to the CI test suite? If so, it seems there's two main issues causing failing CI runs: 
* Quality style check. Running `make style` and pushing the changes should resolve these.
* Unexpected change in behaviour in one of our custom tokenizers. It seems that some of the tests for `JumanppTokenizer` are now failing as a result of the changes in this PR. 

For the CI runs, you should be able to click on `Details` for each of the CI runs to see the full output including error messages / failed test reports. Let us know if this isn't working for you. 

<|||||>> 

Yes, I mean `CI test suite`.

- Where to run `make style`?
- How to fix this? Should I reopen another PR for this commit?

Thanks!<|||||>@jacklanda 
> Where to run make style?

In the top level of your local `transformers` fork. This and more information about how to contribute to the library can found in the [contributor guideline](https://github.com/huggingface/transformers/blob/main/CONTRIBUTING.md#start-contributing-pull-requests).

> How to fix this? Should I reopen another PR for this commit?

The changes should be part of this PR and pushed to this branch. However, I can see that this PR has been opened on the `main` branch of your fork, and not a new feature branch. For this PR we can still merge the changes, however if you wish to contribute again to transformers using this fork it may become tricky to manage conflicts. I would suggest deleting this branch once this PR is merged, fetching `main` from the parent repository and then working from feature branches. Again, see the contributor guidelines for details on typical workflows for PRs. 