Update Reducing Memory Consumption guide with more details

[sergiopaniego]

What does this PR do?

Updated the Reducing Memory Consumption guide with some missing techniques and some additional context (linking to transformers resources).

Before submitting

• 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,
  Pull Request section?
• Was this discussed/approved via a GitHub issue? Please add a link
  to it if that's the case.
• Did you make sure to update the documentation with your changes?
• Did you write any new necessary tests?

Who can review?

@albertvillanova @qgallouedec @kashif

[Copilot]

Pull Request Overview

This PR enhances the "Reducing Memory Consumption" guide by expanding it from a placeholder into a comprehensive documentation resource. The update adds missing memory optimization techniques, provides clearer guidance on when each technique applies, and links to external resources for additional strategies.

Key Changes:

• Added new documentation sections for padding sequences and gradient checkpointing
• Improved organization by consolidating repeated code snippets and adding clearer technique prerequisites
• Enhanced existing sections with more precise language and better cross-references

---

_{Tip: Customize your code reviews with copilot-instructions.md. Create the file or learn how to get started.}

[HuggingFaceDocBuilderDev]

The docs for this PR live here. All of your documentation changes will be reflected on that endpoint. The docs are available until 30 days after the last update.

[albertvillanova]

Great! Thanks. 🤗

[qgallouedec]

Cool! Just a small comment

[qgallouedec]

I don't think you need FA for this one