refactor: merge output_path and filepath into output_dir

[ydcjeff]

fix #46

Description:

• merged output_path and filepath into output_dir
• output_dir is then modified into output_dir / the time right now
• checkpoints will be stored in output_dir / checkpoints
• python logging will be stored in output_dir / training-info.log
• tensorboard outputs will be stored in output_dir

Tree view

output_dir |– 20210407-000000 |– training-info.log |– checkpoints |– best_model_9.pt |– tensorboard_logs |– ... |– 20210407-010000 |– training-info.log |– checkpoints |– best_model_19.pt |– tensorboard_logs |– ... 

[vfdev-5]

Looks good @ydcjeff , i would just preprend the time with some discrimative config words, e.g.

- 20210407-000000 + resnet18-20210407-000000

otherwise, in practice, we have to enter each folder to get the actual config.
It can be also practical for TB when we'd like to filter experiments.