FARMReader should convert local hf-transformers model to FARM if necessary

[mathislucka]

Proposed changes: Before this change, only remote models in huggingface transformers format could be used by the FARMReader. Sometimes, people might have stored these locally and still want to use them with a FARMReader. This PR infers if the local model is in hf or FARM format from the presence of a FARM specific file and thus allows to also use local models in hf format.

Status (please check what you already did):

• [ ] First draft (up for discussions & feedback)
• [x ] Final code
• [ ] Enable actions on your fork
• [ ] Added tests
• [ ] Updated documentation

[julian-risch]

Hi @mathislucka what's the current state of this PR? Should we start the review process or is this a draft? Could you please merge the current master branch in either case? Thank you! 🙏

[mayankjobanputra]

Closing this as it is being fixed by #3663 .