Use cache_dir() instead of hardcoded ~/.cache path

[antimora]

Replace dirs::home_dir().join(".cache") with dirs::cache_dir() for cross-platform compatibility. This respects platform conventions and environment variables like $XDG_CACHE_HOME on Linux.

The codebase was using dirs::home_dir().join(".cache") which is Linux-specific. This PR switches to dirs::cache_dir() which returns the platform-appropriate cache directory:

Platform	Path
Linux	$XDG_CACHE_HOME or ~/.cache
macOS	~/Library/Caches
Windows	{FOLDERPATH_LOCAL_APPDATA}

Pull Request Template

Checklist

• Confirmed that cargo run-checks command has been executed.
• Made sure the book is up to date with changes in this PR.

Related Issues/PRs

N/A

Changes

• Replace dirs::home_dir().join(".cache") with dirs::cache_dir() for cross-platform compatibility
• Update documentation to reflect the change

Testing

Tested mnist example to make sure the cache is saved under ~/Library/Caches on mac

Breaking

Can redownload or alert missing files in ~/.cache path

[laggui]

Seems reasonable.

Though it will break the current cache for macOS and Windows users, and linux users that set $XDG_CACHE_HOME. So we will have to note the breaking change.

[codecov]

Codecov Report

❌ Patch coverage is 77.77778% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 68.87%. Comparing base (3cd0671) to head (1fb7d44).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
crates/burn-dataset/src/vision/mnist.rs	0.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4372      +/-   ##
==========================================
+ Coverage   68.86%   68.87%   +0.01%     
==========================================
  Files        1412     1412              
  Lines      168245   168243       -2     
==========================================
+ Hits       115857   115874      +17     
+ Misses      52388    52369      -19     

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