Add testing

[Alicegaz]

Support two types of testing:

1. Running with the separate script: Add Test DataModule and Test TaskModule to support testing models on a separate script.
2. Manually restricting dataset types and Lightning Module loop types to be executed by running the train.py: Add a parameter --mode to train.py in order to allow for train or validate (and maybe test) options. As the user may want to just run validation for benchmarking with no training using the training configs, Data Modules, and Task Modules, it would be good to add a parameter to allow skipping training and executing validation only (or test only). It may look like that:

if args.mode=="train":
     trainer.fit(model, dataloaders=data_module)
elif args.mode=="valid":
     trainer.validate(model, dataloaders=data_module)

In this case, we require all train and valid (and maybe) data loaders to be initialized.