[BUG] CuTe DSL >= 4.6: FlashAttention-4 SM100 backward with block-sparse masks spills registers, up to 1.8x slower than 4.5.3 (same kernel source)

[BUG] CuTe DSL >= 4.6: FlashAttention-4 SM100 backward with block-sparse masks spills registers, up to 1.8x slower than 4.5.3 (same kernel source) #2660