[ET-VK] Add etvk.use_existing_vma config to avoid duplicate VMA symbols

Pull Request resolved: #18522

Apps that already link VulkanMemoryAllocatorInstantiated (e.g. Stella, via IGL
or Diamond/Skia) get duplicate symbol errors when also linking ExecuTorch's
Vulkan backend, because vma_api.cpp defines VMA_IMPLEMENTATION independently.

Add a Buck config flag `etvk.use_existing_vma=1` that:
- Defines ETVK_USE_META_VMA, which makes vma_api.h match the third-party
  VulkanMemoryAllocatorInstantiated config (Vulkan 1.2, dynamic function
  loading) so struct layouts agree
- Skips VMA_IMPLEMENTATION in vma_api.cpp so no duplicate definitions are
  emitted
- Swaps the Buck dep from VulkanMemoryAllocator_xplat (header-only) to
  VulkanMemoryAllocatorInstantiated (pre-compiled)

Off by default — no behavior change for existing builds or OSS.
ghstack-source-id: 364855830
@exported-using-ghexport

Differential Revision: [D98250268](https://our.internmc.facebook.com/intern/diff/D98250268/)

Author: ssjia

backends/vulkan/runtime/vk_api/memory/vma_api.cpp

@@ -6,5 +6,7 @@
  * LICENSE file in the root directory of this source tree.
  */
 
+#ifndef ETVK_USE_META_VMA
 #define VMA_IMPLEMENTATION
+#endif
 #include <executorch/backends/vulkan/runtime/vk_api/memory/vma_api.h>

backends/vulkan/runtime/vk_api/memory/vma_api.h

@@ -13,6 +13,32 @@
 // Always include this file (vma_api.h) instead.
 //
 
+#ifdef ETVK_USE_META_VMA
+
+// Match the config from the shared third-party
+// VulkanMemoryAllocatorInstantiated library so that struct layouts agree.
+// We do NOT define VMA_IMPLEMENTATION here; the third-party static lib
+// provides it.
+#undef VMA_STATIC_VULKAN_FUNCTIONS
+#undef VMA_DYNAMIC_VULKAN_FUNCTIONS
+#define VMA_STATIC_VULKAN_FUNCTIONS 0
+#define VMA_DYNAMIC_VULKAN_FUNCTIONS 1
+#define VMA_VULKAN_VERSION 1002000
+
+#ifdef __clang__
+#pragma clang diagnostic push
+#pragma clang diagnostic ignored "-Wnullability-completeness"
+#pragma clang diagnostic ignored "-Wunused-variable"
+#endif /* __clang__ */
+
+#include <vk_mem_alloc.h>
+
+#ifdef __clang__
+#pragma clang diagnostic pop
+#endif /* __clang__ */
+
+#else // !ETVK_USE_META_VMA
+
 #define VMA_VULKAN_VERSION 1000000
 
 #ifdef USE_VULKAN_WRAPPER
@@ -56,3 +82,5 @@
 #ifdef __clang__
 #pragma clang diagnostic pop
 #endif /* __clang__ */
+
+#endif // ETVK_USE_META_VMA

backends/vulkan/targets.bzl

@@ -69,6 +69,10 @@ def get_vulkan_preprocessor_flags(no_volk, is_fbcode):
         if debug_mode:
             VK_API_PREPROCESSOR_FLAGS += ["-DVULKAN_DEBUG"]
 
+    vma_dep = read_config("etvk", "vma_dep", "xplat")
+    if vma_dep == "instantiated":
+        VK_API_PREPROCESSOR_FLAGS += ["-DETVK_USE_META_VMA"]
+
     if force_no_extensions:
         VK_API_PREPROCESSOR_FLAGS += ["-DETVK_FORCE_NO_EXTENSIONS"]
 
@@ -152,6 +156,7 @@ def vulkan_spv_shader_lib(name, spv_filegroups, is_fbcode = False, no_volk = Fal
 
 def define_common_targets(is_fbcode = False):
     debug_mode = read_config("etvk", "debug", "0") == "1"
+    vma_dep = read_config("etvk", "vma_dep", "xplat")
 
     runtime.python_library(
         name = "gen_vulkan_spv_lib",
@@ -185,9 +190,14 @@ def define_common_targets(is_fbcode = False):
 
         suffix = "_no_volk" if no_volk else ""
 
-        VK_API_DEPS = [
-            "fbsource//third-party/VulkanMemoryAllocator/3.2.0:VulkanMemoryAllocator_xplat",
-        ]
+        if vma_dep == "instantiated":
+            VK_API_DEPS = [
+                "fbsource//third-party/VulkanMemoryAllocator/3.0.1:VulkanMemoryAllocatorInstantiated",
+            ]
+        else:
+            VK_API_DEPS = [
+                "fbsource//third-party/VulkanMemoryAllocator/3.2.0:VulkanMemoryAllocator_xplat",
+            ]
 
         default_deps = []
         android_deps = ["fbsource//third-party/toolchains:android"]