Reintroduce generate method for PPOTrainer

[CloseChoice]

What does this PR do?

In the release v0.12.0 the generate and the _generate_batched methods were removed from the PPOTrainer. As there is feedback from the community (see the issues #3250 and #3270) (including a member's comment) this PR reintroduces the method and tests it properly. Note that the reintroduced methods are basically copy-pasted from version v0.11.4 and then slightly adapted to the current code.

Supports #3250 (not strictly closing since in the reproducible example the .step method is used as well, which this PR does not reintroduce).

Before submitting

• [ ] This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
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Who can review?

Anyone in the community is free to review the PR once the tests have passed. Feel free to tag
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[CloseChoice]

Just realized that there is the generate_completions method and the batch_generation function already. I would assume that reusing this functionality should be the target of this PR, not duplicating functionality as it is done right now. Though right now this is not possible since the generate_completions method does not return anything. Would it be suitable to add a return to the generate_completions?

[qgallouedec]

What do you think is the best?

[CloseChoice]

What do you think is the best?

I would prefer reusing functionality if this does not add much complexity and doesn't break existing tests. I'll prepare an update to the generate_completions function and update this PR, will ping you once that is ready for a first review.

[CloseChoice]

@qgallouedec I updated the functionality and reused the generate method that we have already. I think this is way cleaner and simpler. My current implementation of generate does not match the one in 0.11.4 exactly, since I wanted to keep it simple. Let me know what you think.