Parameterize the top_k_categorical_accuracy metric

[somnathrakshit]

Modify to have parameterized version of accuracy metric

[jstypka]

Hey @somnathrakshit, thanks for the PR!

This change makes parametrisation of the top_k_accuracy metric possible, but doesn't enable us to do anything else. The way I thought about it, is to have the top_custom_categorical_accuracy function in the user code, outside of Magpie and enable a field called metrics=[] in the Magpie API to pass arbitrary functions to be computed. That way users can compute whatever they want. We should also enable some sensible defaults to be computed when nothing is passed in.

Hope that makes sense!

[somnathrakshit]

Yes, it does. I will try to do it over the weekend but not sure if I will get time or not.