v6.8.0: SELU layer, attention, improved GPU/CPU compatibility

✨ Major features and improvements

• Add SELU layer, from Klambauer et al. (2017).
• Add parametric soft attention layer, as in Yang et al. (2016).
• New higher-order function uniqued, which wraps layers giving them a per-batch cache.
• Improve batch normalization, by tracking activation moving averages.

🔴 Bug fixes

• Fix GPU usage in pooling operations.
• Add optimized code for extracting ngram features.
• Improve CPU/GPU compatibility.
• Improve compatibility of LinearModel class.

👥 Contributors

Thanks to @tammoippen for the pull request!