Multiple fixes preparing for multi-gpu

Fix arange to avoid overflow.

Better error message

Rework amdlibm exclusion on GPU.

Otherwise it is possible to get into a situation where we use the dnn flags, but the cuda ones aren't defined and stuff breaks.

Also starts adressing issue #3468 a bit.

Speedup scan grad() method

renaming tensor4d to tensor4 in stack examples

Use make_new_inplace mechanism to fix optimization with cast