We recently updated flash attention and have more cu files. Each cu file need a lot of CPU memory to compile.

Previously, we did not set nvcc_threads, and number_of_nvcc_threads() returns 3 for A10 build machine. That number is too large since the memory is limited (55 GB), while number of parallel is large. For example, if the machine has 8 cpu cores, 8 * 3 means there are total 24 nvcc threads. It might encounter out of memory.

Here we update the number_of_nvcc_threads() to use updated number of flash attention cu files, and explicitly set nvcc_threads in CI build to avoid out-of-memory in build.