Add semantic segmentation (linear) code (#185)

dinov2/eval/segmentation/__init__.py

dinov2/eval/segmentation/hooks/__init__.py

dinov2/eval/segmentation/hooks/optimizer.py

dinov2/eval/segmentation/models/__init__.py

dinov2/eval/segmentation/models/backbones/__init__.py

.../eval/segmentation/models/backbones/vision_transformer.py

dinov2/eval/segmentation/models/decode_heads/__init__.py

dinov2/eval/segmentation/models/decode_heads/linear_head.py

dinov2/eval/segmentation/utils/__init__.py

dinov2/eval/segmentation/utils/colormaps.py

notebooks/semantic_segmentation.ipynb