Trainer registers subjects with Metric

[mochic]

Trainer currently initializes subjects with Metric when calling register_subject. Our starting policy for our curriculum operates on our own Metric subclass which forces us to include code that handles this edgecase in our initialization policy.

[mochic]

This might not be necessary if we calculate metrics at trainer evaluation time (pulling from slims). I also kinda only encounter this problem due to my current implementation but after switching to the implementation @dyf mentioned, this will probably not be an issue for me.