Migrate to transformers v5 (blocked by HF issue #43638)

[BobTheInnkeeper]

We need to migrate this repo to transformers v5.

Migration work branch:

• https://github.com/EPFLiGHT/MultiMeditron/tree/transformers-migration

Related upstream issue (likely blocker for the migration):

• IndexError: index 0 is out of bounds for dimension 0 with size 0 with deepspeed zero3 traininig and a non pretrained Bert model huggingface/transformers#43638

The upstream issue needs to resolve in some way for us to migrate to transformers v5.