[Feat] Add Vit-Det as LW-DETR encoder

[felixdittrich92]

This PR:

• Add Vit-Det as classification model (encoder for LW-DETR)
• Add corresponding tests
• Add docs entry
• Add preps for layout models (onnx export / device and dtype set)
• Add ChannelLayerNorm which will be required by LW-DETR

[felixdittrich92]

Related to #2059

[codecov]

Codecov Report

❌ Patch coverage is 94.90741% with 11 lines in your changes missing coverage. Please review.
✅ Project coverage is 96.43%. Comparing base (db9d92a) to head (aa68b04).

Files with missing lines	Patch %	Lines
doctr/models/modules/layers/pytorch.py	28.57%	10 Missing ⚠️
...tr/models/classification/vit_det/layers/pytorch.py	98.68%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2063      +/-   ##
==========================================
- Coverage   96.48%   96.43%   -0.05%     
==========================================
  Files         142      146       +4     
  Lines        6111     6319     +208     
==========================================
+ Hits         5896     6094     +198     
- Misses        215      225      +10     

Flag	Coverage Δ
unittests	96.43% <94.90%> (-0.05%)	⬇️

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