scikit-learn

* PR to 7288
Use _BaseComposition as base

*  Fix flakes problem

* Change ``pipeline``, add more tests and other changes
1. Use ``_BaseComposition`` in class ``Pipeline`` and ``FeatureUnion``
2. Add tests of soft voting ``transform`` when one estimator is set to None
3. Add estimator name validation in ``_BaseComposition`` and tests
4. Other requested changes.

* Remove the unused import warn

* Add more test and documentation

* resolve conflict with master

* Add testing cases and modify documentation

* Add to whats_new.rst

*  Fix too many blank lines