Update docs with new Lightning Lite usage 1/n

[awaelchli]

What does this PR do?

We introduced a new way to interact with Lightning Lite in #14992. The run method approach is no longer the recommended way moving forward.

This first PR focuses on

• Updates to remove subclassing LightningLite in example code
• self.x -> lite.x
• Usage of the CLI

Future PRs will need to

• Add section on how to run in Jupyter notebook
• Update the animated GIF

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Did you have fun?

I made sure I had fun coding 🙃

cc @Borda @carmocca @justusschock @awaelchli

[carmocca]

This effectively forces us to release lite on the next minor release. It would be weird to have it in the docs if we are going to have large breaking changes by changing the names. Is this agreed on?

[Borda]

cc: @lantiga

[awaelchli]

This PR doesn't change any names or breaks APIs. I'm updating the docs based on what we have in master. Old code will still work. I am quite confused about what the issue here is. Where would you expect this documentation to show up if not here?

[carmocca]

I'm just saying that since this PR changes the docs to use the new package and imports, then whoever is reading the docs will see this. Right now it's only on the "latest" docs, but not anymore after we make a 1.x release which means that it would be messy to change names after that. This is also related to the motivation in #15586 (comment)

[awaelchli]

Oh I see. I think you mean the changes I made from

:class:~pytorch_lightning.lite.LightningLite
to
:class:~lightning_lite.lite.LightningLite
?
So this is not visible to the user in the docs, as it gets rendered as just LightningLite with a link. The rest of the changes should only be about importing from lightning the main package. If it's just about that, I can also revert that.

[awaelchli]

Reverted the link changes. Now still points to :class:~pytorch_lightning.lite.LightningLite

[tchaton]

LGTM !