[MINOR] [MLLIB] rename some functions of PythonMLLibAPI

Keep the same naming conventions for PythonMLLibAPI.
Only the following three functions is different from others
```scala
trainNaiveBayes
trainGaussianMixture
trainWord2Vec
```
So change them to
```scala
trainNaiveBayesModel
trainGaussianMixtureModel
trainWord2VecModel
```
It does not affect any users and public APIs, only to make better understand for developer and code hacker.

Author: Yanbo Liang <ybliang8@gmail.com>

Closes #7011 from yanboliang/py-mllib-api-rename and squashes the following commits:

771ffec [Yanbo Liang] rename some functions of PythonMLLibAPI