Add linear learning rate scheduler

[astral4]

Checklist

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

Related Issues/PRs

#1198

Changes

This PR adds a linear learning rate scheduler to Burn, which didn't exist previously. This makes progress towards the issue linked above.

Testing

I wrote tests to confirm that invalid config parameters are rejected and that the learning rate changes over iterations as expected. I successfully ran run-checks.sh all. I also checked the appearance of the new documentation via cargo doc --open.

[codecov]

Codecov Report

Attention: Patch coverage is 82.81250% with 11 lines in your changes are missing coverage. Please review.

Project coverage is 85.82%. Comparing base (56f4602) to head (5d03879).
Report is 4 commits behind head on main.

❗ Current head 5d03879 differs from pull request most recent head 9b4d35d. Consider uploading reports for the commit 9b4d35d to get more accurate results

Files	Patch %	Lines
crates/burn-core/src/lr_scheduler/linear.rs	82.81%	11 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1443      +/-   ##
==========================================
+ Coverage   85.76%   85.82%   +0.06%     
==========================================
  Files         640      644       +4     
  Lines       70632    71563     +931     
==========================================
+ Hits        60577    61420     +843     
- Misses      10055    10143      +88     

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[laggui]

Thanks for working on this! 🙏

Overall, LGTM. Just a minor comment.

[laggui]

Sorry I missed some comments in the last review since it is retroactive to the change of step_size to f64. The changes were kind of implicit but that's my fault 😅

See my comments below, and then LGTM 🚀

[astral4]

Alright, it should be resolved now. Thanks!