Use :emphasize-lines: in sphinx docs to highlight code.

Nobody has claimed this yet.

## 📚 Documentation

For typos and doc fixes, please go ahead and:

```py
.. code-block:: python
:emphasize-lines: 3,5

def some_function():
interesting = False
print 'This line is highlighted.'
print 'This one is not...'
print '...but this one is.'
```

______________________________________________________________________

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- [**Metrics**](https://github.com/PyTorchLightning/metrics): Machine learning metrics for distributed, scalable PyTorch applications.

- [**Lite**](https://pytorch-lightning.readthedocs.io/en/latest/starter/lightning_lite.html): enables pure PyTorch users to scale their existing code on any kind of device while retaining full control over their own loops and optimization logic.

- [**Flash**](https://github.com/PyTorchLightning/lightning-flash): The fastest way to get a Lightning baseline! A collection of tasks for fast prototyping, baselining, fine-tuning, and solving problems with deep learning.

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- [**Lightning Transformers**](https://github.com/PyTorchLightning/lightning-transformers): Flexible interface for high-performance research using SOTA Transformers leveraging Pytorch Lightning, Transformers, and Hydra.

cc @borda @rohitgr7 @tchaton @akihironitta

Search the Sphinx documentation for code blocks that should highlight selected lines, using the example in the issue as the reference. Add the :emphasize-lines: directive to the relevant blocks and confirm the rendered documentation highlights the intended lines.

Written by the indexing model from the issue text.