Parallelize SPIR-V compilation (#4200)

Summary:
Pull Request resolved: #4200

Due to a high number of shaders to compile, shader compilation has become quite slow (taking around 1 minute from empirical observations). This change updates the SPIR-V compilation script to parallelize SPIR-V compilation over the available number of CPU cores.

Testing the impact of parallelization using the following command:

```
buck build //xplat/executorch/backends/vulkan:gen_vulkan_graph_runtime_shaderlib_cpp --out ~/scratch/shaders
```

We can observe the following improvement:

| | Before | After|
|--- | --- | --- |
| M1 Mac | 42.4s | 7.8s |

Reviewed By: liuk22, jorgep31415

Differential Revision: D59597186

fbshipit-source-id: 72df756a3a1f818688af66bbce4fc9fc02f1f505

Author: SS-JIA

backends/vulkan/runtime/gen_vulkan_spv.py

@@ -15,6 +15,7 @@
 import re
 import sys
 from itertools import product
+from multiprocessing.pool import ThreadPool
 
 sys.path.append(os.path.abspath(os.path.join(os.path.dirname(__file__), "..")))
 import subprocess
@@ -620,9 +621,12 @@ def constructOutputMap(self) -> None:
 
     def generateSPV(self, output_dir: str) -> Dict[str, str]:
         output_file_map = {}
-        for shader_name in self.output_shader_map:
-            source_glsl = self.output_shader_map[shader_name][0]
-            shader_params = self.output_shader_map[shader_name][1]
+
+        def process_shader(shader_paths_pair):
+            shader_name = shader_paths_pair[0]
+
+            source_glsl = shader_paths_pair[1][0]
+            shader_params = shader_paths_pair[1][1]
 
             with codecs.open(source_glsl, "r", encoding="utf-8") as input_file:
                 input_text = input_file.read()
@@ -652,9 +656,15 @@ def generateSPV(self, output_dir: str) -> Dict[str, str]:
                 ]
 
                 print("glslc cmd:", cmd)
-                # pyre-ignore
                 subprocess.check_call(cmd)
 
+                return (spv_out_path, glsl_out_path)
+
+        # Parallelize shader compilation as much as possible to optimize build time.
+        with ThreadPool(os.cpu_count()) as pool:
+            for spv_out_path, glsl_out_path in pool.map(
+                process_shader, self.output_shader_map.items()
+            ):
                 output_file_map[spv_out_path] = glsl_out_path
 
         return output_file_map