if anyone else faces a long delay before the training starts: disable group_by_length
in trainer_pt_utils.py line 506, this is called:
 lengths = [len(feature[self.model_input_name]) for feature in dataset]
which iterates over the whole dataset without use of dataloader workers. this took a pretty long time in my case.
In the long run in might be worth to submit a pull request to change this behavior, or add a flag to enable getting the lengths using a dataloader?