segregate nlp and non-nlp (cv) namespace in evalem

[NISH1001]

What

We need to segregate existing evalem namespace into different categories based on downstream tasks such as NLP, CV, etc.
Eg: evalem.nlp, evalem.cv...

How

For this to work, we need to have model wrappers, processors, et. at the right level of abstraction so that downstream namespaces could easily be differentiated using base/parent class types in the implementation.

[NISH1001]

Related thread at:
#19

[rbavery]

addressed by #27

[thesujitroy]

adding NLP , restructuring the evalm - adding losses (DICE

[NISH1001]

The new restructuring with evalem.nlp and evalem.cv has passed the final tests at:
#27 (comment)

This PR has been merged into main branch now.