Add `multiclass` fine-tuning example

[qubvel]

It would be great to have more examples for fine-tuning in the library!

Similar to current examples for binary-segmentaion and multi-label 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.