Perf: Fusion search for composed optimization

[nathanielsimard]

I added the ability to re-order operations in the execution stream to find more optimizations.

For the custom GELU fusion benchmarks, if I put an unrelated tensor operation in the middle of the computation, I get a 3x slowdown with the old method, since it broke fusion. With these changes, I don't experience any slowdown since the unrelated operation executes out of order.

[codecov]

Codecov Report

Attention: Patch coverage is 89.77177% with 121 lines in your changes missing coverage. Please review.

Project coverage is 82.32%. Comparing base (d8dc28c) to head (3b9d523).
Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
...es/burn-cubecl-fusion/src/shared/trace/executor.rs	17.14%	29 Missing ⚠️
crates/burn-fusion/src/search/merging.rs	91.18%	29 Missing ⚠️
crates/burn-fusion/src/stream/context.rs	60.71%	11 Missing ⚠️
crates/burn-cubecl-fusion/src/base.rs	37.50%	10 Missing ⚠️
...ates/burn-fusion/src/search/optimization/stream.rs	94.81%	10 Missing ⚠️
crates/burn-fusion/src/search/block.rs	95.03%	8 Missing ⚠️
...ates/burn-cubecl-fusion/src/reduce/optimization.rs	33.33%	6 Missing ⚠️
crates/burn-fusion/src/stream/execution/tests.rs	95.34%	4 Missing ⚠️
crates/burn-cubecl-fusion/src/shared/builder.rs	0.00%	3 Missing ⚠️
crates/burn-fusion/src/backend.rs	0.00%	3 Missing ⚠️
... and 4 more

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3258      +/-   ##
==========================================
+ Coverage   82.22%   82.32%   +0.09%     
==========================================
  Files         973      978       +5     
  Lines      124018   124864     +846     
==========================================
+ Hits       101971   102790     +819     
- Misses      22047    22074      +27     

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[laggui]

All CI failures are unrelated to the changes in this PR 🙃

Windows and mac failed because the url is unreachable for the text-generation example tokenizer.

Linux failed on tests::cube::kernel::bernoulli::tests::wald_wolfowitz_runs_test (seems to be failing intermittently since the kernel was ported to cubecl repo).

Gonna re-run the jobs for sanity.