Use word index for determining whether a token is a subword when addi…
Author: timothyjlaurent
Pull request: yes

…ng word labels.

# What does this PR do?
Use repeated word index instead of offset[0] == 0 for assigning pad labels to non-first subwords.

As described in the #17220, words like 그 will be split into 2 subwords both with 0,1 offsets but with the same word index. This PR ensures that this second subword receives the correct -100 label

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Fixes #17220


## 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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- [ ] Did you write any new necessary tests?
I tried to write a test but the test tokenizer doesn't split my test word. Open to suggestions or maybe it can be a slow test with the pretrained tokenizer.

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@NielsRogge
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Comments:
The docs for this PR live [here](https://moon-ci-docs.huggingface.co/docs/transformers/pr_17221). All of your documentation changes will be reflected on that endpoint.

@NielsRogge  do you have suggestions for making a test case for the fast tokenizer that will split the "그" token?

Could I use the pretrained Layoutlmv2 tokenizer and mark the test as slow, or do you have suggestions for how to properly configure a test tokenizer that will split that token?



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