nlp and cv namespace segregation

Disclaimer: This creates a breaking changes but only at namespace level. So, all the previous evalem.models, evalem.metrics, etc are now residing at evalem.nlp.models, evalem.nlp.metrics.

With this new release, now evalem has both nlp as well as cv namespace segregation:

• evalem.nlp
• evalem.cv

Both of these have:

• models
• metrics
• evaluation pipeline

All these are derived from bases at evalem._base.