Reduce the number of variants

[leofang]

Right now, we build 3 variants for the nvmath-python package:

• core
• cuda11
• cuda12

This was done because of the drastic difference in the packaging layout since CUDA 12 (see docs). After CUDA 11 support is dropped (either by conda-forge or by nvmath-python), we should be able to reduce the variants to 2: core and cuda.