Issue Warning when plotting MDS with two few topics

[ajdapretnar]

Feature request

I propose having a nice explainable warning that informs the user one cannot plot MDS with insufficient number of topics.

Motivation

The issue was opened many times: #1914, #1010, #725, #378. This means people get confused by the original error. Having an integrated error issued after a check on the number of topics in the topic model would be beneficial.

Your contribution

I looked into the code, but honestly didn't know where such a warning would be most appropriate and how to format it.

[MaartenGr]

If I'm not mistaken, you would like to see an error raised when .visualize_topics() is used with too few topics, right?

In that case, I think the best place to do this would be here:

https://github.com/MaartenGr/BERTopic/blob/master/bertopic/plotting/_topics.py

There, we can check how many topics the topic model has and raise an error before doing any other calculations.