Add naive MultiHeadFFN

[AntonOresten]

I was able to get a forward pass working that was ~20% faster, with no intermediate activations, but we already have the chunked StarGLU, and I would rather wait for something like cuTile in Julia to get actual good performance.

[codecov]

Codecov Report

❌ Patch coverage is 0% with 13 lines in your changes missing coverage. Please review.
✅ Project coverage is 31.20%. Comparing base (2279585) to head (df5d4d6).

Files with missing lines	Patch %	Lines
src/transformers/MultiHeadFFN.jl	0.00%	13 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #43      +/-   ##
==========================================
- Coverage   31.72%   31.20%   -0.53%     
==========================================
  Files          36       37       +1     
  Lines         769      782      +13     
==========================================
  Hits          244      244              
- Misses        525      538      +13     

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