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.
Merge pull request #1888 from nouiz/gpu_sqr_sum_ax0

Add GpuSqrSumAx0 to lower the memory usage on the GPU.