Add exporting model section to the Colab

## 📚 Documentation Issue

The official Colab hasn't export part, it's a huge lack of documentation for newcomers.

* [Link to the official colab](https://colab.research.google.com/drive/16jcaJoc6bCFAQ96jDe2HwtXj7BMD_-m5).

* Google Colab now uses PyTorch ≥1.8, that supports exporting.

Start with the official Colab linked in the issue and review its current training and inference sections. Add an export-model section using the PyTorch ≥1.8 capability mentioned in the issue, then verify that a newcomer can follow it and that the notebook runs.

Written by the indexing model from the issue text.