ghstack-source-id: 4132ec7
Pull-Request-resolved: #159695

In some cases we have mps kernels which are reused across higher-order-op subgraphs and the toplevel code. However, currently we initialize the variable for the mps kernel the first time we use it, which runs into an issue if we run into the mps kernel within a subgraph since the kernel will only be initialized within the subgraph scope. For instance:
```
if ...
    auto mps_lib_0_func = ...
    mps_lib_0_func->run()

// since we already used mps_lib_0 once, we don't re-initialize it
mps_lib_0_func->run()  // error, mps_lib_0_func not initialized
```

So the solution we took here is to initialize all the kernels at the beginning:
```
const std::shared_ptr<at::native::mps::MetalKernelFunction> get_mps_lib_0() {
    static const auto func = mps_lib_0.getKernelFunction("generated_kernel");
    return func;
}
AOTIMetalKernelFunctionHandle get_mps_lib_0_handle() {
    static const auto handle = AOTIMetalKernelFunctionHandle(get_mps_lib_0().get());
    return handle;
}
...
if ...
    get_mps_lib_0()->run()

get_mps_lib_0()->run()  // success
```

Pull Request resolved: #159753
Approved by: https://github.com/malfet
ghstack dependencies: #159456, #159695