refactor dialogue state tracking for modelling/dataset interoperability
Signed-off-by: Zhilin Wang <zhilinw@nvidia.com>

Design based on attached diagram
[UML proposed dialogue1.pdf](https://github.com/NVIDIA/NeMo/files/7946212/UML.proposed.dialogue1.pdf)

Currently supports SGD-QA (Bert-based) and Huggingface GPT2 models, and planning to support other models. 