Add CRF module

[Sun-Xiaohui]

Conditional Random Field (CRF) is a basic model of natural language processing, which is widely used in word segmentation, named entity recognition, part-of-speech tagging and other tagging scenarios. For example, BiLSTM-CRF model is widely used in NER. Could Flax add CRF module? That will useful! Thanks

[chiamp]

Flax currently tries to keep the framework minimal, adding only essential layers that can serve as building blocks for more complex layers and layers that are very commonly used. For everything else, we encourage users to fork our repo and create their own custom layers.