[GPT-2] Add docs
# What does this PR do?

Lots of people don't seem to know about batched generation with GPT-2 and friends. Hence this PR adds a section to the docs, similar to the T5 docs.

It also fixes an issue in the T5 docs.
cc @stevhliu <|||||>The docs for this PR live [here](https://moon-ci-docs.huggingface.co/docs/transformers/pr_24432). All of your documentation changes will be reflected on that endpoint.<|||||>This issue has been automatically marked as stale because it has not had recent activity. If you think this still needs to be addressed please comment on this thread.

Please note that issues that do not follow the [contributing guidelines](https://github.com/huggingface/transformers/blob/main/CONTRIBUTING.md) are likely to be ignored.<|||||>@stevhliu can we proceed by adding this, or would you like to have it on a separate page?<|||||>I think it'll be best to put this in its own section on the [Text generation strategies](https://huggingface.co/docs/transformers/generation_strategies#text-generation-strategies) page :)