ggml: Enable flash attention for vision encoders

Although the vision component of multimodal models typically already
call the optimized nn.Attention, it is converted into non-fused
operations. That is because the backend-specific fused kernels may
have requirements, such as padding, and they is performed by the
cache, which vision encoders don't use.

This implements a fallback path in the backend, softening the
requirements into optimizations. In turn, this allows flash attention
to be used for vision encoders, saving a significant amount of VRAM
and improving performance.

ml/backend.go

@@ -233,8 +233,10 @@ type Tensor interface {
 //
 // kqv := value.Mulmat(ctx, kq)
 // return kqv.Permute(ctx, 0, 2, 1, 3).Contiguous(ctx)
+//
+// cacheConfigApplied indicates whether the optimizations requested through CacheConfig have been performed
 type ScaledDotProductAttention interface {
-	ScaledDotProductAttention(ctx Context, key, value, mask, sinks Tensor, vmla Tensor, scale float64) Tensor
+	ScaledDotProductAttention(ctx Context, key, value, mask, sinks Tensor, vmla Tensor, scale float64, cacheConfigApplied bool) Tensor
 }
 
 type number interface {