Revert "Refactor. Use decorators for skipping cuda-only-tests."

There is no more use for choosing single tests to skip.

This reverts commit 99295012bafccf637b6ad846c9f8719f4ebca1d1.
Fix typo.
Add config flag experimental.unpickle_gpu_on_cpu.

Allows unpickling gpu cudaNdarray as numpy.ndarray.
See AddConfigVar comment for more details.