[Discussion] Use control-flow operators (e.g. foreach) to unroll dynamic graph models

[eric-haibin-lin]

apache/mxnet#11948 introduces foreach to the contrib.unroll method which makes RNNs hybridizable and easy to export to json for deployment. In theory it introduces extra overhead such as calling nd.where. We need to evaluate the performance impact (I assume it's relatively small) and change gluon nlp language models to hybrid blocks for ease of deployment if possible. It should not affect existing pre-trained models since the parameter structure won't change.

@zheng-da @szha