v6.6.0: Improved GPU usage and examples

✨ Major features and improvements

• Add GPU kernels for max and mean pool using variable-length sequences.
• thinc.api.FeatureExtractor, for getting features from spaCy Doc objects.

🔴 Bug fixes

• Improve multi-device handling
• thinc.api.add now accepts a variable number of layers.
• Improve Residual class.

⚠️ Backwards incompatibilities

• Some of the example code may be out of date.

📖 Documentation and examples

• Add reader for WikiNER corpora.
• Add example for Twitter NER.
• Add Siamese network example.