Add example for extracting representation.

[maclandrol]

Changelogs

This PR has a very minor documentation updates to add example of how to leverage SAFE-GPT representation (see context in #18).

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Checklist:

• Add tests to cover the fixed bug(s) or the new introduced feature(s) (if appropriate).
• Update the API documentation if a new function is added, or an existing one is deleted. Eventually consider making a new tutorial for new features.
• Write concise and explanatory changelogs below.
• If possible, assign one of the following labels to the PR: feature, fix or test (or ask a maintainer to do it for you).

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It should be noted that the model was not trained with any representation learning objective. As such using embedding from SAFE-GPT is not recommended. However, I will be happy to contribute to any attempt at building a new pretrained version with a training objective taking that into account.