Implement global shader registry (#2222)

Summary:
X-link: pytorch/pytorch#121088


## Context

This changeset updates Vulkan SPIR-V codegen to introduce a global SPIR-V shader registry and register shaders dynamically at static initialization time. This change makes it possible to define and link custom shader libraries to the ATen-Vulkan runtime.

Before:

* `gen_vulkan_spv.py` generated two files, `spv.h` and `spv.cpp` which would contain the definition and initialization of Vulkan shader registry variables. 


After:

* Introduce the `ShaderRegistry` class in `api/`, which encapsulates functionality of the `ShaderRegistry` class previously defined in the generated `spv.h` file
* Introduce a global shader registry (defined as a static variable in the `api::shader_registry() function`
* Define a `ShaderRegisterInit` class (taking inspiration from `TorchLibraryInit`) that allows for dynamic shader registration
* `gen_vulkan_spv.py` now only generates `spv.cpp`, which defines a static `ShaderRegisterInit` instance that triggers registration of the compiled shaders to the global shader registry.

Benefits:

* Cleaner code base; we no longer have `ShaderRegistry` defined in a generated file, and don't need a separate implementation file (`impl/Registry.*`) to handle shader lookup. All that logic now lives under `api/ShaderRegistry.*`
* Makes it possible to compile and link separate shader libraries, providing similar flexibility as defining and linking custom ATen operators

Differential Revision: D54447700

Author: SS-JIA

backends/vulkan/targets.bzl

@@ -53,8 +53,8 @@ def define_common_targets():
             "@EXECUTORCH_CLIENTS",
         ],
         exported_deps = [
-            "//caffe2:torch_vulkan_api",
             "//caffe2:torch_vulkan_ops",
+            "//caffe2:torch_vulkan_spv",
         ],
         define_static_target = False,
     )