Sum gathered input tokens
Author: techkang
Pull request: yes

# What does this PR do?
Fix https://github.com/huggingface/transformers/issues/34503
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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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## Who can review?

Anyone in the community is free to review the PR once the tests have passed. Feel free to tag
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Models:

- text models: @ArthurZucker
- vision models: @amyeroberts, @qubvel
- speech models: @ylacombe, @eustlb
- graph models: @clefourrier

Library:

- flax: @sanchit-gandhi
- generate: @zucchini-nlp (visual-language models) or @gante (all others)
- pipelines: @Rocketknight1
- tensorflow: @gante and @Rocketknight1
- tokenizers: @ArthurZucker
- trainer: @muellerzr and @SunMarc
- chat templates: @Rocketknight1

Integrations:

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- ray/raytune: @richardliaw, @amogkam
- Big Model Inference: @SunMarc
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HF projects:

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- 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)

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Comments:
cc @muellerzr 

The docs for this PR live [here](https://moon-ci-docs.huggingface.co/docs/transformers/pr_34554). All of your documentation changes will be reflected on that endpoint. The docs are available until 30 days after the last update.