update doc for perf_train_cpu_many, add intel mpi introduction
Signed-off-by: Wang, Yi A <yi.a.wang@intel.com>

# What does this PR do?

update doc for perf_train_cpu_many, add intel mpi introduction

Fixes # (issue)


## Before submitting
- [x] This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
- [ ] Did you read the [contributor guideline](https://github.com/huggingface/transformers/blob/main/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/main/docs), and
      [here are tips on formatting docstrings](https://github.com/huggingface/transformers/tree/main/docs#writing-source-documentation).
- [ ] Did you write any new necessary tests?


## Who can review?

Documentation: @sgugger

@sgugger please help review, thanks very much<|||||>_The documentation is not available anymore as the PR was closed or merged._<|||||>Failure is unrelated to this PR (would disappear with a rebase) so merging. Thanks again!