Fix dummy objects for quantization
# What does this PR do?

The dummy objects introduced recently are actually not generated and maintained by the dummy scripts, because the `is_xxx_available` test they rely on contains an _ in the xxx. This PR fixes that.
Another fix concerns the models `ForNextSentencePrediction` which were not included in the list yet (a better fix will use the model mapping names from the auto mapping one day when I have more time :-) )

This is blocking the doc building for the new frontend so merging as soon as it's green.
Thanks for taking care of it!