Added a MultiCPU SLURM example using Accelerate Launch and MPIRun

[okhleif-IL]

What does this PR do?

This PR adds a multicpu SLURM example in the examples/slurm folder using Accelerate and MPIRun. This script can run a multicpu SLURM training example on the complete_nlp_example.py script.

This PR also fixes a minor typo in all other SLURM example .sh scripts regarding activating the environment.

Before submitting

• [x] This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
• [x] Did you read the contributor guideline, Pull Request section?
• [ ] Was this discussed/approved via a Github issue or the forum? Please add a link to it if that's the case.
• [x] Did you make sure to update the documentation with your changes? Here are the documentation guidelines, and here are tips on formatting docstrings.
• [x] Did you write any new necessary tests?

Who can review?

Anyone in the community is free to review the PR once the tests have passed. Feel free to tag members/contributors who may be interested in your PR.

[HuggingFaceDocBuilderDev]

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