[Bugfix][Vulkan] Call VulkanDeviceAPI destructor on program exit

Most of the TVM Global() functions allocate with "new" and do
not deallocate, as the OS can clean up any leftover buffers at
the end.  In this case, we need the VulkanDeviceAPI destructor
to call vkDestroyInstance, to prevent a segfault on exit when
using some nvidia drivers.

src/runtime/vulkan/vulkan.cc

@@ -412,8 +412,13 @@ class VulkanDeviceAPI final : public DeviceAPI {
   }
 
   static VulkanDeviceAPI* Global() {
-    static VulkanDeviceAPI* inst = new VulkanDeviceAPI();
-    return inst;
+    // Most of the TVM Global() functions allocate with "new" and do
+    // not deallocate, as the OS can clean up any leftover buffers at
+    // the end.  In this case, we need the VulkanDeviceAPI destructor
+    // to call vkDestroyInstance, to prevent a segfault on exit when
+    // using some nvidia drivers.
+    static VulkanDeviceAPI inst;
+    return &inst;
   }
 
   const VulkanContext& context(size_t device_id) const {