Enforce MvStudentT nu to be a scalar variable

[ricardoV94]

#5173 revealed a bit of an edge case in the MvStudentT with broadcasting when nu is a vector. Since we haven't tested whether the RV, logp, and logcdf, are doing the right thing in that case, we should assert that nu is a scalar in the make_node method of the respective RandomVariable.

We can always remove this restriction if we confirm things are working consistently with the rules of broadcasting.

[sagartomar]

I would like to work on this. It looks like we need to override the make_node method in MvStudentTRV class and need to check if nu is scalar and if the check passes then call the super class' make_node method.

[twiecki]

Great, go for it @sagartomar 👍.