ml: Abstract attention out of model definitions

There are two benefits to doing this:
 - Provide a library function that models can use, reducing code for
   each model implementation
 - Enables a single place to drop in optimized implementations of
   attention based on the backend or other factors. One is provided for
   GGML.

On CUDA this improves token generation rate by about 3%. It does not
have a significant effect on Metal.

Co-authored-by: Daniel Hiltgen <daniel@ollama.com>

ml/nn/attention.go

@@ -0,0 +1,59 @@
+package nn
+
+import (
+	"fmt"
+
+	"github.com/ollama/ollama/ml"
+)
+
+// Attention implements scaled dot-product attention for transformer models:
+// Attention(Q, K, V) = softmax(QK^T/√d_k)V
+//
+// Parameters:
+//   - ctx: Context for tensor operations
+//   - query: Query tensor (Q) with shape [d_k, seq_len_q, heads]
+//   - key: Key tensor (K) with shape [d_k, seq_len_k, kv_heads]
+//   - value: Value tensor (V) with shape [seq_len_k, d_v, kv_heads]
+//   - mask: Optional attention mask that is added to the attention score. If
+//     provided, should broadcast to [seq_len_k, seq_len_q, heads]
+//   - scale: Scaling factor, typically 1/√d_k where d_k is the key dimension
+//
+// Returns:
+//
+//	Attention output with shape [d_v, heads, seq_len_q]
+func Attention(ctx ml.Context, query, key, value, mask ml.Tensor, scale float64) ml.Tensor {
+	if query.Dim(0) != key.Dim(0) {
+		panic(fmt.Errorf("d_k in attention operation does not match between query(%v) and key(%v)", query.Dim(0), key.Dim(0)))
+	}
+
+	if mask != nil && query.Dim(1) != mask.Dim(1) {
+		panic(fmt.Errorf("seq_len_q in attention operation does not match between query(%v) and mask(%v)", query.Dim(1), mask.Dim(1)))
+	}
+
+	if key.Dim(1) != value.Dim(0) {
+		panic(fmt.Errorf("seq_len_k in attention operation does not match between key(%v) and value(%v)", key.Dim(1), value.Dim(0)))
+	}
+
+	if mask != nil && key.Dim(1) != mask.Dim(0) {
+		panic(fmt.Errorf("seq_len_k in attention operation does not match between key(%v) and mask(%v)", key.Dim(1), mask.Dim(0)))
+	}
+
+	if key.Dim(2) != value.Dim(2) {
+		panic(fmt.Errorf("kv_heads in attention operation does not match between key(%v) and value(%v)", key.Dim(2), value.Dim(2)))
+	}
+
+	if sdpa, ok := query.(ml.ScaledDotProductAttention); ok {
+		return sdpa.ScaledDotProductAttention(ctx, key, value, mask, scale)
+	} else {
+		kq := key.MulmatFullPrec(ctx, query)
+
+		kq = kq.Scale(ctx, scale)
+		if mask != nil {
+			kq = kq.Add(ctx, mask)
+		}
+		kq = kq.Softmax(ctx)
+
+		kqv := value.Mulmat(ctx, kq)
+		return kqv.Permute(ctx, 0, 2, 1, 3).Contiguous(ctx)
+	}
+}