CUDA: Improve flash decoding kernel occupancy for BS=1 case

[gaugarg-nv]

For BS=1, the gen phase flash_attn_vec_ext_f32 kernel is launched with a constant parallel_blocks value of 4. Check code.
However, parallel_blocks = 4 causes poor occupancy on GPU.

Consider following models. The current occupancy is far below what is achievable if parallel_blocks value is increased.

Model	num_heads	head_dim	occupancy with PB=4 on RTX 4090	achievable occupancy with optimal PB value on RTX 4090
Llama 3B	24	128	0.06	0.25
Llama 8B	32	128	0.08	0.25
Qwen 1.5B	12	128	0.03	0.25
Qwen 7B	28	128	0.07	0.25

I have a change that addresses this issue and it shows improvement in gen phase performance by up to 14%.