[ET-VK][ez] Don't copy zeros for cache tensors

Currently, cache tensors for SDPA are prepacked even though the mutable buffer data just contains zeros.

For fused SDPA, this step can be skipped.

Differential Revision: [D86226137](https://our.internmc.facebook.com/intern/diff/D86226137/)

[ghstack-poisoned]

Author: ssjia

backends/vulkan/runtime/graph/ops/glsl/sdpa_kv_cache_update.glsl

@@ -14,6 +14,10 @@ ${define_required_extensions(DTYPE)}
 
 layout(std430) buffer;
 
+#define DEBUG_MODE
+
+#extension GL_EXT_debug_printf : enable
+
 #include "common.glslh"
 
 ${layout_declare_tensor(B, "w", "t_cache", DTYPE, OUTPUT_STORAGE, is_scalar_array=False)}
@@ -80,13 +84,17 @@ void main() {
   const int S = projected_sizes.z;
   const int H = projected_sizes.y;
 
-  if (d4 >= D4 || s >= S || h >= H) {
+  const int c = s + input_pos; // idx along max_context_len dim
+  const int C = cache_sizes.z;
+
+  if (d4 >= D4 || c >= C || h >= H) {
     return;
   }
 
-  const int c = s + input_pos; // idx along max_context_len dim
-  const int C = cache_sizes.y;
+  IN_VEC4_T in_texel = IN_VEC4_T(0.0);
+  if (s < S) {
+    in_texel = read_projected_d4(d4, h, s, D4, H, S);
+  }
 
-  IN_VEC4_T in_texel = read_projected_d4(d4, h, s, D4, H, S);
   write_cache_d4(in_texel, d4, c, h, D4, C, H);
 }

backends/vulkan/runtime/graph/ops/impl/SDPA.cpp

@@ -575,9 +575,9 @@ void compute_attn_weight_with_kv_cache_impl(
 
   utils::StorageType cache_storage = graph.storage_type_of(q_projected);
   const ValueRef k_cache =
-      prepack_standard(graph, k_cache_data, cache_storage, utils::kWidthPacked);
+      graph.add_tensor_like(k_cache_data, cache_storage, utils::kWidthPacked);
   const ValueRef v_cache =
-      prepack_standard(graph, v_cache_data, cache_storage, utils::kWidthPacked);
+      graph.add_tensor_like(v_cache_data, cache_storage, utils::kWidthPacked);
 
   update_cache_impl(graph, {k_projected, k_cache, input_pos_symint, -1});
   update_cache_impl(graph, {v_projected, v_cache, input_pos_symint, -1});