Add ChainEval Class

[mstfatay]

Right now there is only one evaluation class which is LLMEval. This class can only evaluate Langchain's LLM objects. I suppose we need to add support for langchain's Chain objects too because they are the most largely used langchain objects.

I suppose we can inherit both LLMEval and ChainEval from a new class called BaseEval.

[iterix2]

This would be a fantastic addition! Thank you for taking out time to raise this @mstfatay !