Fix test in FAST_COMPILE.
Don't put all specialize opt in fast_compile_gpu!

This fix buildbot tests in FAST_COMPILE.
Fix test in FAST_COMPILE as now we register more opt in fast_compile.
Doc COMPUTE_GPU_MEM_USED
[DOC] small doc update.
Merge pull request #2153 from nouiz/cuda_tests

[BUILDBOT CRASH] Fix test in FAST_COMPILE.