apputils/checkpoint.py: load_checkpoint can be called w/o specifying the model

This is inspired by @barrh’s PR https://github.com/NervanaSystems/distiller/pull/246
but it at a “slower-integration-pace” and w/o changing APIs.

1. create_model() attaches model attributes (arch, dataset, is_parallel) to created models.
2. save_checkpoint() stores the new model attributes with checkpoint metadata
3. load_checkpoint() can be invoked with model=None, in which case we attempt
to create the model from the stored checkpoint metadata.