RETRO model finetuning
# What does this PR do ?

1. Add RETRO model NQA finetune scripts.
2. Make RETRO model compatible with MegatronLM RETRO model.
    - Can turn off the RoPe positional embedding.
    - Add version to RETRO model.
3. Add stride to the RETRO indexed memory map file.
4. during inference, host multiple BERT models for retrieval service to use.