Fix to include day count in training duration for metrics.json  (#2718)
Fix to include day count in training duration for metrics.json  (#2718)

* Added default predictor for bimpm model

* Fix #2717: Add day count in training duration

This fixes Issue [@2717](https://github.com/allenai/allennlp/issues/2717) to include day count in `training_duration` key in metrics.

* Modify elapsed time format to use `timedelta`

`time.strftime` does not account for number of days more than
31. Changing it to `datetime.timedelta` and using its `str`
representation for printing epoch duration as well as training
duration.

allenai/allennlp,allenai/allennlp,allenai/allennlp,allenai/allennlp