text: Title: Add `multiclass` fine-tuning example
  Body: It would be great to have more examples for fine-tuning in the library!
  
  Similar to current examples for [binary-segmentaion](https://github.com/qubvel-org/segmentation_models.pytorch/blob/main/examples/binary_segmentation_intro.ipynb) and [multi-label](https://github.com/qubvel-org/segmentation_models.pytorch/blob/main/examples/cars%20segmentation%20(camvid).ipynb) segmentation would be great to have multi-class segmentation example (the whole image is annotated). The same camvid dataset can be used.
  
  The example should showcase how to 
   - fine-tune a model with pytorch-ligtning (or any other fine-tuning framework, even a plain pytorch)
   - use loss function with ignore index for unlabeled class
   - compute metrics per-class
   - visualize results
  
  In case anyone wants to work on this you are welcome! Just notify in this issue, and then ping me to review a PR when it's ready.