Feat/autodiff/checkpoint ops

[louisfd]

Finally, a real PR for autodiff checkpointing.
There is now a way to do no checkpointing by considering everything to be compute bound. It is set by default, so this should be able to merge without impacting anything.

[codecov]

Codecov Report

Attention: Patch coverage is 82.68377% with 351 lines in your changes are missing coverage. Please review.

Project coverage is 85.60%. Comparing base (cbf7550) to head (79eab9d).
Report is 1 commits behind head on main.

Files	Patch %	Lines
crates/burn-autodiff/src/ops/module.rs	69.91%	108 Missing ⚠️
crates/burn-autodiff/src/ops/tensor.rs	88.02%	100 Missing ⚠️
crates/burn-autodiff/src/checkpoint/builder.rs	62.94%	73 Missing ⚠️
...ates/burn-autodiff/src/checkpoint/retro_forward.rs	45.94%	20 Missing ⚠️
crates/burn-autodiff/src/checkpoint/state.rs	77.21%	18 Missing ⚠️
crates/burn-autodiff/src/checkpoint/strategy.rs	42.85%	16 Missing ⚠️
crates/burn-autodiff/src/checkpoint/base.rs	62.16%	14 Missing ⚠️
crates/burn-autodiff/src/ops/activation.rs	98.36%	1 Missing ⚠️
crates/burn-autodiff/src/tensor.rs	96.55%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1358      +/-   ##
==========================================
- Coverage   85.69%   85.60%   -0.09%     
==========================================
  Files         586      592       +6     
  Lines       65339    66836    +1497     
==========================================
+ Hits        55993    57216    +1223     
- Misses       9346     9620     +274     

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