* 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
* parallelized VotingClassifier
* rename list to list_ to avoid problems
* Added new tests for sample_weight, multithreading and multiprocessing
* assert_equal -> assert_array_equal
* Fixed sample_weight existence check and test_sample_weight
* Code is clearer now
* Tests refactoring, 'backend' parameter removed
* Tests indentation fix
* reverted parallel predict and predict_proba to single threaded version
* what's new section added
* minor fixes
* check for sample_weight support in underlying estimators added
* newline at the end of test
Yichuan Liu