fix: replace assert with ValueError for llm validation

[iw4p]

It's absolutely better to use raise ValueError instead of assert. (or maybe someone has a better idea)
If you agree for the next refactors I can implement this method

def check_llm(self):
    if self.llm is None:
        raise ValueError("LLM must be set before using this method")

So by using this method whenever we need to use llm, we can easily call it before that line like
self.check_llm()

[jjmachan]

thanks a lot for the suggestion and the PR 🙂 ❤️. hopefully with #1558 we will be able to fix this but keeping it for v0.3 so as to not break anything

closing this for now but I'm really sorry we couldn't merge it 🙁 but at the same time thanks a million for taking the time to raise this, really grateful too and do checkout this form https://docs.google.com/forms/d/e/1FAIpQLSdM9FrrZrnpByG4XxuTbcAB-zn-Z7i_a7CsMkgBVOWQjRJckg/viewform - our way of saying thank you 🙂