[autodiff] Handle mutable for loop index

[erizmr]

Issue: #7771

Brief Summary

🤖 Generated by Copilot at 30ef20b

This pull request enhances the automatic differentiation (AD) pass to handle more complex for loops. It fixes some aliasing and consistency issues in taichi/transforms/auto_diff.cpp, and adds a new test case in tests/python/test_ad_for.py to verify the correctness of the AD pass on nested and mutable loop indices.

Walkthrough

🤖 Generated by Copilot at 30ef20b

• Add a method to check if a statement is a mutable loop index and modify the condition for promoting statements to local variables in the AD pass (link, link)
• Add a method to mark local variables that are used as loop boundaries as needing AD stack allocation (link)
• Add a logic to clone and insert AD stack load statements before the loop execution in the adjoint program (link)
• Add a test case to test_ad_for.py to check the correctness of the AD pass on nested for loops with mutable indices (link)

[netlify]

✅ Deploy Preview for docsite-preview ready!

Name	Link
🔨 Latest commit	f737baf
🔍 Latest deploy log	https://app.netlify.com/sites/docsite-preview/deploys/6433cfb2b2056a00088e2cfa
😎 Deploy Preview	https://deploy-preview-7778--docsite-preview.netlify.app
📱 Preview on mobile	Toggle QR Code... Use your smartphone camera to open QR code link.

---

To edit notification comments on pull requests, go to your Netlify site settings.