vulkan: Set limit for task concurrency (#5427)

A common default for the maximum number of open files is 256, which can
lead to `asyncio.gather(*tasks)` failing with Too many open files.

    $ python ggml_vk_generate_shaders.py --glslc=$ANDROID_NDK_PATH/shader-tools/darwin-x86_64/glslc
    ggml_vulkan: Generating and compiling shaders to SPIR-V
    Traceback (most recent call last):
      File "/Users/neuman/Code.noindex/github/llama.cpp/ggml_vk_generate_shaders.py", line 2326, in <module>
        asyncio.run(main())
      File "/Users/neuman/Code.noindex/miniforge3/lib/python3.10/asyncio/runners.py", line 44, in run
        return loop.run_until_complete(main)
      File "/Users/neuman/Code.noindex/miniforge3/lib/python3.10/asyncio/base_events.py", line 649, in run_until_complete
        return future.result()
      File "/Users/neuman/Code.noindex/github/llama.cpp/ggml_vk_generate_shaders.py", line 2294, in main
        await asyncio.gather(*tasks)
    [...snip...]
    OSError: [Errno 24] Too many open files

This change sets a reasonable concurrency limit for tasks (and therefore
open files), without significant impact on run time.

ggml_vk_generate_shaders.py

@@ -2067,6 +2067,8 @@ type_names = {
 
 K_QUANTS_PER_ITERATION = 2
 
+ASYNCIO_CONCURRENCY = 64
+
 output_dir = gettempdir()
 
 lock = asyncio.Lock()
@@ -2291,7 +2293,14 @@ async def main():
     tasks.append(string_to_spv("rope_neox_f32", rope_neox_src, {"A_TYPE": "float", "D_TYPE": "float"}))
     tasks.append(string_to_spv("rope_neox_f16", rope_neox_src, {"A_TYPE": "float16_t", "D_TYPE": "float16_t"}))
 
-    await asyncio.gather(*tasks)
+    # Helper to decorate tasks with semaphore acquisition.
+    async def withSemaphore(sem, task):
+        async with sem:
+            return await task
+
+    # Run tasks concurrently guarded by a concurrency limit.
+    sem = asyncio.Semaphore(ASYNCIO_CONCURRENCY)
+    await asyncio.gather(*(withSemaphore(sem, task) for task in tasks))
 
     with open("ggml-vulkan-shaders.hpp", "w") as f:
         f.write("#include <cstdint>\n\n")