[vulkan] Add VMA as a third_party subrepo (#83906)
the [VulkanMemoryAllocator](https://github.com/GPUOpen-LibrariesAndSDKs/VulkanMemoryAllocator) is a popular library for GPU memory allocation using Vulkan. The Vulkan backend has a dependency on it, but since it is only a single header file we currently include it by checking it into the repo under [aten/src/ATen/native/vulkan/api/vk_mem_alloc.h](https://github.com/pytorch/pytorch/blob/master/aten/src/ATen/native/vulkan/api/vk_mem_alloc.h). However, it is better to check it in as a third party submodule, since it allows better version tracking.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/83906
Approved by: https://github.com/kimishpatel
