fine tuning existing dpr model #8

Hi, Thanks for posting a nice repo!
I see that we can train the DPR model with GC-DPR but I guess we need to train it from scratch by loading base models (bert-base-uncased|roberta-base)
How can we make use of this repo to fine tune a pretrained DPR model. For example, we already have DPR encoder models provided by Facebook.
question_model = "facebook/dpr-question_encoder-single-nq-base"
context_model = "facebook/dpr-ctx_encoder-single-nq-base"

To make these models domain-specific my idea is to fine-tune these models with domain data.
It would be helpful if you can let me know how we can load question and context models with train_dense_encoder function.
Any other suggestion would be appreciated.