Fix bert position ids in DPR convert script
https://github.com/huggingface/transformers/commit/614fef1691edb806de976756d4948ecbcd0c0ca3 introduced buffers for position ids for BERT that breaks the DPR convert script since the DPR weights don't have those.

To fix that I followed @LysandreJik 's suggestion to manually add the position ids to the state dict before loading it into the model.