Gluon-nlp: [Model] Port OpenAI GPT to gluon-nlp

[eric-haibin-lin]

Port the GPT model to gluon-nlp:
https://github.com/openai/gpt-2
https://github.com/openai/finetune-transformer-lm

A good first step is to port their pre-trained models and be able to perform inference.

[sxjscience]

I've ported the pretrained model. Codes: https://github.com/sxjscience/gluonnlp-gpt2

[szha]

the model is merged in the model zoo in #761. we still need to move it to the API. when moving, we need to:

• reuse existing transformer class (and convert the model correspondingly)
• add documentation
• make sure it's flexible enough to plug into other use cases such as using it as a feature encoder