[ET-VK] Refactor Context::flush() and make ComputeGraph destructor exception-safe (#17191)

Refactored Context::flush() by extracting two new public functions:
- wait_for_queue(): blocks until the GPU queue is idle
- clear_resources(): clears command/descriptor pools and cleanup lists

This allows callers to use these operations independently. ComputeGraph's
destructor now uses these functions directly and wraps them in try/catch
blocks to ensure it never throws exceptions, following C++ best practices
for destructors.

Authored with Claude.

Differential Revision: [D92171362](https://our.internmc.facebook.com/intern/diff/D92171362/)

Author: pytorchbot

backends/vulkan/runtime/api/Context.cpp

@@ -226,13 +226,14 @@ void Context::submit_cmd_to_gpu(VkFence fence_handle, const bool final_use) {
   }
 }
 
-void Context::flush() {
+void Context::wait_for_queue() {
   VK_CHECK(vkQueueWaitIdle(queue().handle));
+}
 
+void Context::clear_resources() {
   command_pool_.flush();
   descriptor_pool_.flush();
 
-  // If there is an existing command buffer, invalidate it
   if (cmd_) {
     cmd_.invalidate();
   }
@@ -243,6 +244,11 @@ void Context::flush() {
   images_to_clear_.clear();
 }
 
+void Context::flush() {
+  wait_for_queue();
+  clear_resources();
+}
+
 bool available() {
   return context();
 }

backends/vulkan/runtime/api/Context.h

@@ -232,6 +232,10 @@ class Context final {
     return cmd_;
   }
 
+  void wait_for_queue();
+
+  void clear_resources();
+
   void flush();
 
 #if defined(VK_KHR_pipeline_executable_properties) && \

backends/vulkan/runtime/graph/ComputeGraph.cpp

@@ -183,13 +183,22 @@ ComputeGraph::ComputeGraph(GraphConfig config)
 }
 
 ComputeGraph::~ComputeGraph() {
-  values_.clear();
+  // Wait for all currently executing commands to complete before cleaning up.
+  // If wait_for_queue() throws an exception, still proceed with cleanup.
+  try {
+    context_->wait_for_queue();
+  } catch (...) {
+  }
 
-  prepack_nodes_.clear();
-  execute_nodes_.clear();
-  clear_deferred_cmds();
+  // Wrap in try/catch to ensure that destructor does not throw
+  try {
+    values_.clear();
 
-  context_->flush();
+    prepack_nodes_.clear();
+    execute_nodes_.clear();
+    clear_deferred_cmds();
+  } catch (...) {
+  }
 }
 
 std::vector<int64_t> ComputeGraph::extract_int_or_symint_list(