perf: avoid redundant MOSS QKV and value layout copies (#2209)

Co-authored-by: leejet <leejet714@gmail.com>

src/core/ggml_extend.cpp

@@ -678,21 +678,23 @@ ggml_tensor* ggml_ext_attention_ext(ggml_context* ctx,
             // CUDA FA MMA starts at 64 channels; keep the original head's attention scale.
             q_in = ggml_pad(ctx, q_in, static_cast<int>(64 - d_head), 0, 0, 0);
             k_in = ggml_pad(ctx, k_in, static_cast<int>(64 - d_head), 0, 0, 0);
+            v_in = ggml_pad(ctx, v_in, static_cast<int>(64 - d_head), 0, 0, 0);
         }
         if (kv_scale != 1.0f) {
             k_in = ggml_ext_scale(ctx, k_in, kv_scale);
         }
         k_in = ggml_cast(ctx, k_in, GGML_TYPE_F16);
 
-        v_in = ggml_ext_cont(ctx, ggml_permute(ctx, v_in, 0, 2, 1, 3));
-        v_in = ggml_reshape_3d(ctx, v_in, d_head, L_k, n_kv_head * N);
-        if (pad_head) {
-            v_in = ggml_pad(ctx, v_in, static_cast<int>(64 - d_head), 0, 0, 0);
-        }
         if (kv_scale != 1.0f) {
             v_in = ggml_ext_scale(ctx, v_in, kv_scale);
         }
-        v_in = ggml_cast(ctx, v_in, GGML_TYPE_F16);
+        v_in       = ggml_permute(ctx, v_in, 0, 2, 1, 3);
+        auto v_f16 = ggml_cast(ctx, v_in, GGML_TYPE_F16);
+        if (!ggml_backend_supports_op(backend, v_f16)) {
+            v_f16 = ggml_cast(ctx, ggml_ext_cont(ctx, v_in), GGML_TYPE_F16);
+        }
+        v_in = v_f16;
+        v_in = ggml_reshape_3d(ctx, v_in, v_in->ne[0], L_k, n_kv_head * N);
 
         if (mask_in != nullptr) {
             // ggml_flash_attn_ext expects the mask as a contiguous F16 tensor shaped

src/model/diffusion/moss_image.hpp

@@ -124,7 +124,8 @@ namespace MOSSImage {
             y           = ggml_add(c, ggml_add(c, y, ggml_mul(c, y, part(1))), part(0));
             auto qkv    = linear("proj.qkv", y);
             auto split  = [&](int i) {
-                return ggml_reshape_4d(c, ggml_ext_slice(c, qkv, 0, i * config.dim, (i + 1) * config.dim), config.head_dim, config.dim / config.head_dim, x->ne[1], x->ne[2]);
+                return ggml_view_4d(c, qkv, config.head_dim, config.dim / config.head_dim, x->ne[1], x->ne[2],
+                                    config.head_dim * qkv->nb[0], qkv->nb[1], qkv->nb[2], i * config.dim * qkv->nb[0]);
             };
             auto q       = norm("q_norm", split(0));
             auto k       = norm("k_norm", split(1));