In a work around, warn about the problem so that the real cause get fixed.

The added test currently fail.
Make SliceConstant to have its signature hashable.
Remove internal Theano stack level from variable creating. This make them more readable to the user.
Create fast_compile_gpu. Reusing gpu tag for nnet opt don't work.

This is because if we exclude the 'gpu' tag, we don't want those opt to also be enabled.
Merge pull request #2140 from nouiz/mixed

[ENH] Add SliceConstant, better stack trace, fast_compile_gpu