Generate: `group_beam_search` requires `diversity_penalty>0.0`
# What does this PR do?

While revisiting `group_beam_search` to review https://github.com/huggingface/transformers/pull/24407, I noticed that we do not require `diversity_penalty` to be `>0.0`. If it is not `>0.0`, then `group_beam_search` degenerates to `beam_search` with `num_beams=num_beams/num_beam_groups` -- users exploring the method risk not seeing its potential.

With this exception, we ensure the degeneration case does not happen (and possibly nudge the users towards the docs)
_The documentation is not available anymore as the PR was closed or merged._