Add Acelerator API to ddp_tutorial

[jafraustro]

Motivation

• Refine the DDP tutorial to make it device-agnostic by using the Accelerator API.

Description

• Integrate the Accelerator API into the DDP tutorial to support multiple accelerators and improve backend initialization.
• Align the implementation with this example from the PyTorch examples repository.

Checklist

• The issue that is being fixed is referred in the description (see above "Fixes #ISSUE_NUMBER")
• Only one issue is addressed in this pull request
• Labels from the issue that this PR is fixing are added to this pull request
• No unnecessary issues are included into this pull request.

[pytorch-bot]

🔗 Helpful Links

🧪 See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/tutorials/3508

• 📄 Preview Python docs built from this PR

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As of commit 29bf2b8 with merge base 120068e ():
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[sekyondaMeta]

@fegin could you take a look at this when you get a chance