enable average tokens across devices
Author: techkang
Pull request: yes

# What does this PR do?

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Fix [https://github.com/huggingface/transformers/issues/34242](https://github.com/huggingface/transformers/issues/34242)


## Before submitting
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@muellerzr pls help to check.
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Comments:
The docs for this PR live [here](https://moon-ci-docs.huggingface.co/docs/transformers/pr_34373). All of your documentation changes will be reflected on that endpoint. The docs are available until 30 days after the last update.

@muellerzr @SunMarc I fixed one typo to make ruff happy and pass auto check. And added more explanation to the argument. Pls check again, thank you.

One required tests failed but it's due to timeout. Can any one help to fix it? Thanks a lot.

world_size is unavailable when only using TF backend. Using try/except to catch errors.

CI still enconunted four errors, but the same error appeared at the recently merged PR. So it may not caused by this PR.

CI errors of this PR: [ci_error](https://app.circleci.com/pipelines/github/huggingface/transformers/108706/workflows/6ef932a9-5847-4736-b178-836b2e1407ba/jobs/1445381/tests)

CI errors for former [PR](https://github.com/huggingface/transformers/pull/34377): [ci_error](https://app.circleci.com/pipelines/github/huggingface/transformers/108644/workflows/ccfcf63a-13d3-4e43-a7fd-16fb3c16ead8/jobs/1444457/tests)

We'll merge once our CI allows us to 😅 

cc @ydshieh 

Thanks @techkang 🤗