Multimodal: Add Cross-Attention and Mixture-of-Transformers Archs

[amazloumi]

This PR is just merging PRs #77 and #78 to main.

[codecov]

Codecov Report

❌ Patch coverage is 95.03106% with 16 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
kempnerforge/model/mot.py	93.85%	4 Missing and 3 partials ⚠️
kempnerforge/model/init.py	81.25%	1 Missing and 2 partials ⚠️
kempnerforge/model/transformer.py	95.16%	1 Missing and 2 partials ⚠️
kempnerforge/distributed/parallel.py	0.00%	1 Missing and 1 partial ⚠️
kempnerforge/model/vlm.py	94.44%	1 Missing ⚠️

Files with missing lines	Coverage Δ
kempnerforge/config/vlm.py	100.00% <100.00%> (ø)
kempnerforge/model/cross_attention.py	100.00% <100.00%> (ø)
kempnerforge/model/modality.py	100.00% <100.00%> (ø)
kempnerforge/model/vlm.py	98.96% <94.44%> (-1.04%)	⬇️
kempnerforge/distributed/parallel.py	58.33% <0.00%> (-0.71%)	⬇️
kempnerforge/model/init.py	90.00% <81.25%> (-10.00%)	⬇️
kempnerforge/model/transformer.py	93.67% <95.16%> (+0.60%)	⬆️
kempnerforge/model/mot.py	93.85% <93.85%> (ø)

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