Cube: Vectorization + simple matmul implementation

[louisfd]

In this PR:

• Adequate vectorization support
• New simple matmul written with Cube. Vectorized on inputs in the dimension K
• Mutability info in Variable analysis
• Fix assignation:
  • For expand element, a new variable is always used if inplace is not available and it does not come from a Comptime function. This removes potential bugs with mutability
  • Other structs can define their own init behaviour, most of them being do nothing

[codecov]

Codecov Report

Attention: Patch coverage is 69.62233% with 185 lines in your changes missing coverage. Please review.

Project coverage is 86.10%. Comparing base (4b174a8) to head (fb3c519).

Files	Patch %	Lines
crates/burn-jit/src/kernel/matmul/simple.rs	32.89%	51 Missing ⚠️
crates/burn-cube/src/frontend/element/int.rs	0.00%	32 Missing ⚠️
crates/burn-cube/src/frontend/element/float.rs	41.17%	20 Missing ⚠️
crates/burn-cube/src/frontend/element/uint.rs	40.62%	19 Missing ⚠️
crates/burn-cube/tests/language/assign.rs	86.52%	19 Missing ⚠️
...rates/burn-cube/src/frontend/element/vectorized.rs	40.00%	9 Missing ⚠️
.../burn-cube-macros/src/codegen_function/function.rs	86.44%	8 Missing ⚠️
crates/burn-cube/src/frontend/comptime.rs	58.82%	7 Missing ⚠️
crates/burn-cube/src/frontend/element/base.rs	33.33%	6 Missing ⚠️
crates/burn-cube-macros/src/analysis.rs	75.00%	5 Missing ⚠️
... and 4 more

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1866      +/-   ##
==========================================
- Coverage   86.24%   86.10%   -0.14%     
==========================================
  Files         774      776       +2     
  Lines       90159    90496     +337     
==========================================
+ Hits        77759    77924     +165     
- Misses      12400    12572     +172     

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