text: Title: Add multi-gpu training example script
  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-GPU training example. This can be handled with [PyTorch-Lightning](https://lightning.ai/docs/pytorch/stable/accelerators/gpu_intermediate.html) or with pure Pytorch.
  
  It might be a `.py` script or Jupyter notebook. The example should showcase how to
  
   - fine-tune a model with pytorch-lightning (or any other fine-tuning framework, even a plain pytorch)
   - compute metrics with correct gathering of results (maybe [torchmetrics](https://lightning.ai/docs/torchmetrics) can be utilized)
   - 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.
  
  Fixes: 
   - https://github.com/qubvel-org/segmentation_models.pytorch/issues/903
   - https://github.com/qubvel-org/segmentation_models.pytorch/issues/896