Finetune guide for semantic segmentation (#18640)
* 📝 first draft
* oops add to toctree
* make style
* 📝 add inference section
* 🖍 make style
* 📝 add images
* 🖍 apply feedbacks
* remove num_labels and pytorch block
* apply feedbacks, add colab notebook
Co-authored-by: Steven <stevhliu@gmail.com>
