In the Python API for Gaussian Mixture Model, predict() and predictSoft() methods should raise an error when the input argument is not an RDD.

Author: FlytxtRnD <meethu.mathew@flytxt.com>

Closes #6180 from FlytxtRnD/GmmPredictException and squashes the following commits:

4b6aa11 [FlytxtRnD] Raise error if the input to predict()/predictSoft() is not an RDD