Patch release: fix bug with PPOTrainer and log_stats

Fixed a bug with log_stats of PPOTrainer to avoid breaking behaviour

• [PPOTrainer] A workaround for failing log_stats by @younesbelkada in https://github.com/huggingface/trl/pull/708

What's Changed

• Release: v0.7.0 by @younesbelkada in https://github.com/huggingface/trl/pull/706
• set dev version by @younesbelkada in https://github.com/huggingface/trl/pull/707

Full Changelog: https://github.com/huggingface/trl/compare/v0.7.0...v0.7.1