text: Title: Add segmentation + classification head 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 example for segmentation + classification head. The same `camvid` or `oxford pet` dataset can be used. As a classification target for these datasets, the "existence" of the class on the image can be used.
  
  The example should showcase how to 
   - fine-tune a model with pytorch-lightning (or any other fine-tuning framework, even a plain pytorch)
   - use loss function for segmentation and classification
   - compute metrics for classification and segmentation heads
   - visualize results
   
  
  [Docs on how to create a model with classification head.](https://smp.readthedocs.io/en/latest/insights.html#aux-classification-output)
  
  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.