[ET-VK][ez] Small fix to SPIR-V compilation artifact caching

## Context

There was a bug in the SPIR-V cache; essentially, once a shader variant is processed it will save the source GLSL to the cache. However, if another variant that uses the same source GLSL then checks the cache, it will think that the source file is unchanged and therefore will not recompile.

To fix this, defer saving the source GLSL files only when all shaders have been compiled.

Differential Revision: [D71973524](https://our.internmc.facebook.com/intern/diff/D71973524/)

ghstack-source-id: 274443433
Pull Request resolved: #9716

Author: SS-JIA

backends/vulkan/runtime/gen_vulkan_spv.py

@@ -801,8 +801,7 @@ def process_shader(shader_paths_pair):
                 output_file.write(output_text)
 
             if cache_dir is not None:
-                # Otherwise, store the generated and source GLSL files in the cache
-                shutil.copyfile(source_glsl, cached_source_glsl)
+                # Otherwise, store the generated GLSL files in the cache
                 shutil.copyfile(glsl_out_path, cached_glsl_out_path)
 
             # If no GLSL compiler is specified, then only write out the generated GLSL shaders.
@@ -852,6 +851,15 @@ def process_shader(shader_paths_pair):
             ):
                 output_file_map[spv_out_path] = glsl_out_path
 
+        # Save all source GLSL files to the cache. Only do this at the very end since
+        # multiple variants may use the same source file.
+        if cache_dir is not None:
+            for _, source_glsl in self.glsl_src_files.items():
+                cached_source_glsl = os.path.join(
+                    cache_dir, os.path.basename(source_glsl) + ".t"
+                )
+                shutil.copyfile(source_glsl, cached_source_glsl)
+
         return output_file_map