* When SWIG_PY is not enabled, paddle.v2 package cannot be used.
* When PYTHON is not enabled, PyDataProvider cannot be used.

Fix issue #1920

* When SWIG_PY is not enabled, paddle.v2 package cannot be used.
* When PYTHON is not enabled, PyDataProvider cannot be used.

Fix issue #1920

cost is Argument, but argument does not have to be cost.

* update function can not be inherited since updateImpl exsits
* remove update implementation in one inherited updater

When the define_py_data_sources2 has both train_list and test_list,
for job=test, the trainer will create both dataProvider_ and testDataProvider_.
But dataProvider_ is not used. This causes SIGSEGV at finishAsync() because asyncLoader_ is not created.

Change-Id: If579f715f80a70ebc795094792c3436bfa0f5746

 * always do test on all test data
 * do test at the end of each pass if test_period=0, otherwise do test if test_period batches passed