Add export/import for all estimators and transformers(which have Scala implementation) under pyspark/ml/regression.py.

yanboliang Please help to review.
For doctest, I though it's enough to add one since it's common usage. But I can add to all if we want it.

Author: Tommy YU <tummyyu@163.com>

Closes #11000 from Wenpei/spark-13033-ml.regression-exprot-import and squashes the following commits:

3646b36 [Tommy YU] address review comments
9cddc98 [Tommy YU] change base on review and pr 11197
cc61d9d [Tommy YU] remove default parameter set
19535d4 [Tommy YU] add export/import to regression
44a9dc2 [Tommy YU] add import/export for ml.regression