Allow deprecated declarations what using Parallel ExecuTorch

Summary:
With the recent BC/FC work by ExecuTorch, many APIs have been marked as deprecated. While the functionality itself is not broken, this results in some build systems treating deprecatd APIs as a failure case.

Specifically `_unsafe_reset_threadpool`

This PR add a pragma allowing for the use of deprecatd APIs

Differential Revision: D64020498

Author: Jack-Khuu

torchao/experimental/ops/parallel-executorch-impl.h

@@ -8,6 +8,17 @@
 
 #include <executorch/extension/threadpool/threadpool.h>
 
+// TODO(T139507292): Remove this once the Executorch API has stabilized. This is
+// here to allow the Executorch team to deprecate APIs in one diff and migrate
+// clients in a following diff. By default, some builds of this file treat
+// deprecated use as an error.
+#ifdef __GNUC__
+#pragma GCC diagnostic warning "-Wdeprecated-declarations"
+#endif
+#ifdef __clang__
+#pragma clang diagnostic warning "-Wdeprecated-declarations"
+#endif
+
 template <typename F>
 void torchao::parallel_1d(const int64_t begin, const int64_t end, const F& f) {
   torch::executorch::threadpool::get_threadpool()->run(