Universal Prompt encoder + Inference Web UI 
# What does this PR do ?

1. Added Perceiver based Universal Prompt encoder.
2. Supports T0 datasets.
3. Added Text generation web server for GPT model and universal prompt encoder.
4. Added Retrieval text generation services and web server. 
5. fixed various bugs for text generation, sentence tokenizer for mGPT and allow data parallel for text generation. 