Extract 'Evaluator' class and a few other changes more

[matomatical]

As part of understanding the script enough to get a sweep running from my machine I was struggling because of how much code there was in the train function, and how model configuration was split between ICLCOnfig defaults and explicit values in get_config function.

• I extracted all evaluations functionality from training function into a new Evaluator class. An Evaluator stores the fixed evaluation batches and provides a method that takes a model and produces the metrics dictionary
  • I have also attempted to inline losses_fn so that all of the evaluation code fits in the one function
• I moved all model and data set configuration into get_config rather than the default values of the ICLConfig class (which now has no default values, so all values should be provided explicitly within get_config)

A small number of other miscellaneous local changes were made (see diff and comments on GitHub PR).

[jqhoogland]

Great I'd had the same idea and am glad you went ahead and dit it