Expose thread count through the parallel interface (pytorch#22908)

## Summary

Expose the active threadpool size through the self-contained
`thread_parallel_interface.h` API. This lets installed-wheel consumers
such as TorchAO use ExecuTorch parallelism without including
`extension/threadpool/threadpool.h`, which is intentionally not shipped
because it exposes the unvendored pthreadpool dependency.

The non-threaded implementation reports one available thread. The
installed SDK link probe now exercises the new symbol.

## Test plan

- `buck2 test
fbsource//xplat/executorch/extension/threadpool/test:thread_parallel_test`
- Built the TorchAO ExecuTorch operator targets against the new
interface
- `git diff --check`
- Python syntax check for the wheel SDK test

AI-assisted: Authored with Codex.

Author: JacobSzwejbka

.ci/scripts/wheel/test_cpp_sdk.py

@@ -1098,7 +1098,8 @@ def test_shipped_headers_have_implementations(work_dir: Path) -> None:
             "#define ET_USE_THREADPOOL\n"
             "#include <executorch/runtime/kernel/thread_parallel_interface.h>\n"
             "using namespace executorch::extension;\n"
-            "int main() { return parallel_for(0, 1, 1, [](int64_t, int64_t) {}) ? 0 : 1; }\n"
+            "int main() { return get_thread_count() > 0 && "
+            "parallel_for(0, 1, 1, [](int64_t, int64_t) {}) ? 0 : 1; }\n"
         ),
     }
 

extension/threadpool/test/thread_parallel_test.cpp

@@ -15,6 +15,7 @@
 #include <executorch/runtime/platform/platform.h>
 
 using namespace ::testing;
+using ::executorch::extension::get_thread_count;
 using ::executorch::extension::parallel_for;
 
 class ParallelTest : public ::testing::TestWithParam<bool> {
@@ -230,6 +231,10 @@ TEST_P(ParallelTest, TestChunkSizeTooLarge) {
   }
 }
 
+TEST(ThreadParallelInterfaceTest, GetThreadCount) {
+  EXPECT_GT(get_thread_count(), 0);
+}
+
 INSTANTIATE_TEST_SUITE_P(
     ParallelTestWithOrWithoutThreadpool,
     ParallelTest,

extension/threadpool/thread_parallel.cpp

@@ -88,5 +88,11 @@ bool parallel_for(
   return true;
 }
 
+int64_t get_thread_count() {
+  return static_cast<int64_t>(
+      ::executorch::extension::threadpool::get_threadpool()
+          ->get_thread_count());
+}
+
 } // namespace extension
 } // namespace executorch

runtime/kernel/thread_parallel_interface.h

@@ -79,6 +79,9 @@ bool parallel_for(
     const int64_t grain_size,
     runtime::FunctionRef<void(int64_t, int64_t)> f);
 
+/** Returns the number of threads available to parallel_for. */
+int64_t get_thread_count();
+
 int64_t get_thread_num();
 
 void set_thread_num(int64_t thread_num);
@@ -96,6 +99,10 @@ inline int64_t get_thread_num() {
   return 0;
 }
 
+inline int64_t get_thread_count() {
+  return 1;
+}
+
 inline void set_thread_num(int64_t thread_num) {
   ET_DCHECK_MSG(false, "cannot set_thread_num without threading support!");
 }