[SPARK-13355][MLLIB] replace GraphImpl.fromExistingRDDs by Graph.apply

`GraphImpl.fromExistingRDDs` expects preprocessed vertex RDD as input. We call it in LDA without validating this requirement. So it might introduce errors. Replacing it by `Graph.apply` would be safer and more proper because it is a public API. The tests still pass. So maybe it is safe to use `fromExistingRDDs` here (though it doesn't seem so based on the implementation) or the test cases are special. jkbradley ankurdave

Author: Xiangrui Meng <meng@databricks.com>

Closes #11226 from mengxr/SPARK-13355.