[ET-VK] Move save_cache from Runtime dtor to model destroy (#7002)

Pull Request resolved: #6974

## Issue
`Runtime` is a local static variable. Hence we'd expect the Runtime dtor to be called on program exit. But on Android devices it's not being invoked. This behavior is different than that seen 6 months ago (D57085281). It's unclear what changed.

This means the cache is not saved due to the following chain never being invoked.
`~Runtime()`  > `~Adapter()` > `~ComputePipelineCache()` > `save_cache()`.

## Solution
Move cache saving to `VulkanBackend.cpp`'s model destroy. This makes sense since the cache is tied to the model and not the runtime.

## Resources
https://medium.com/@martin00001313/mastering-static-objects-in-c-initialization-destruction-and-best-practices-760b17734195
ghstack-source-id: 254701488
@exported-using-ghexport

Differential Revision: [D66179917](https://our.internmc.facebook.com/intern/diff/D66179917/)

---------

Co-authored-by: jorgep31415 <[email protected]>

Author: pytorchbot

backends/vulkan/runtime/VulkanBackend.cpp

@@ -621,6 +621,10 @@ class VulkanBackend final : public ::executorch::runtime::BackendInterface {
   void destroy(DelegateHandle* handle) const override {
     if (handle != nullptr) {
       ComputeGraph* compute_graph = static_cast<ComputeGraph*>(handle);
+      compute_graph->context()
+          ->adapter_ptr()
+          ->compute_pipeline_cache()
+          .save_cache();
       // ComputeGraph is not trivially destructible. Since
       // this was constructed manually in init(), we must destroy it manually
       // here.

backends/vulkan/runtime/vk_api/Pipeline.cpp

@@ -406,8 +406,6 @@ ComputePipelineCache::~ComputePipelineCache() {
     return;
   }
 
-  save_cache();
-
   vkDestroyPipelineCache(device_, pipeline_cache_, nullptr);
   pipeline_cache_ = VK_NULL_HANDLE;
 }

backends/vulkan/runtime/vk_api/Pipeline.h

@@ -269,9 +269,10 @@ class ComputePipelineCache final {
     }
   };
 
+  void save_cache();
+
  private:
   std::vector<char> load_cache();
-  void save_cache();
 
   // Multiple threads could potentially be adding entries into the cache, so use
   // a mutex to manage access