Hi,
I had a similar issue trying to build apex with cuda10.2 and pytorch 1.7 nightly. You can use symbolic linking like the following to resolve the issue:
ln -s /usr/include/cublas_v2.h /usr/local/cuda-10.2/targets/x86_64-linux/include/cublas_v2.h .  Please use appropriate cuda version and target for the target link. Cheers!