[ET-VK] Move save_cache from Runtime dtor to model destroy

## 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

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

[ghstack-poisoned]

Author: jorgep31415

backends/vulkan/runtime/VulkanBackend.cpp

@@ -626,6 +626,7 @@ class VulkanBackend final : public ::executorch::runtime::BackendInterface {
       // here.
       compute_graph->~ComputeGraph();
     }
+    api::context()->adapter_ptr()->compute_pipeline_cache().save_cache();
   }
 };
 

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