add cuda fallback bf16 for compute_cap < 8.0

[haricot]

tested and works with:

nvidia-smi   --query-gpu="compute_cap"  --format=csv
compute_cap
6.1

cargo run -F "cuda,cudnn" --example llama  --release -- --model-id meta-llama/Llama-3.2-1B-Instruct --temperature 0.1 --which v32-1b-instruct --seed 42 --dtype bf16 --prompt

related EricLBuehler#57

[LaurentMazare]

Shouldn't the new code be in some else blocks for the #if __CUDA_ARCH__ >= 800 blocks that contain the proper bf16 implementation?

[haricot]

You are certainly right because I do not encounter the condition CUDA_ARCH >= 800 in this control flow in my case but this must possibly cause a function cuda error already present, I will review this.