Confirm loss and eval_fn()

[plpxsk]

Confirm valid implementation

References:

Loss? Loss is:
Total span extraction loss is the sum of a Cross-Entropy for the start and end positions.

https://huggingface.co/transformers/v4.9.2/model_doc/bert.html?highlight=bertforquestionanswering
https://github.com/huggingface/transformers/blob/v4.44.2/src/transformers/models/bert/modeling_bert.py#L1996

google:
https://github.com/google-research/bert/blob/master/run_squad.py#L646

[plpxsk]

Losses are dropping, and more training means better answers, so seems to be ok for now