Migrate ExecutorchModule.mm to use the new HierarchicalAllocator span ctor (#391)

Summary:
Pull Request resolved: #391

Stop using the deprecated HierarchicalAllocator ctor.
ghstack-source-id: 201128786
exported-using-ghexport

Reviewed By: tarun292

Differential Revision: D49344927

fbshipit-source-id: 8b9b9f00d6c15984f119a680b4393fb9197a4038

Author: dbort

examples/ios_demo_apps/ExecutorchMobileNet/ExecutorchMobileNet/ExecutorchMobileNet/ExecutorchModule.mm

@@ -162,12 +162,8 @@ - (char*)segmentImage:(void*)imageBuffer
   MemoryAllocator temp_allocator{MemoryAllocator(0, nullptr)};
   temp_allocator.enable_profiling("temp allocator");
 
-  MemoryAllocator non_const_allocators[1]{
-      MemoryAllocator(kMemoryAmount, activation_pool)};
-  non_const_allocators[0].enable_profiling("non_const_allocators");
-
-  HierarchicalAllocator non_const_allocator{
-      HierarchicalAllocator(1, non_const_allocators)};
+  Span<uint8_t> non_const_buffers[1]{{activation_pool, kMemoryAmount}};
+  HierarchicalAllocator non_const_allocator({non_const_buffers, 1});
 
   MemoryManager memory_manager{MemoryManager(
       &const_allocator,