Fixed bug where the variable name was defined but not used.
Merge pull request #1889 from AlassaneNdiaye/fix_test_cross_validation_bug

Fixed bug detected by test_cross_validation.py:test_train_cv
Allow import without the gpu back-end.

needed for docgen.
Merge pull request #1893 from nouiz/import

Allow import without the gpu back-end.
Add GpuSqrSumAx0 to lower the memory usage on the GPU.
pep8
small fix.
rm print in test
Allow GpuCAReduce do unary elemwise operation on the input.

The opt to merge the Elemwise and the reduction is limited to sqr, as
otherwise, we need to time, as it could slow things down.
remove code not needed anymore
Make GpuCAReduceCuda support pre_scalar_op
Make opt to use GpuCAReduceCUDA pre_scalar_op with the sqr op.