aio: note the reason for compiling with -O0 (#1249)

Co-authored-by: NOlatunji Ruwase <olruwase@microsoft.com>

op_builder/async_io.py

@@ -30,6 +30,7 @@ class AsyncIOBuilder(OpBuilder):
         return ['csrc/aio/py_lib', 'csrc/aio/common']
 
     def cxx_args(self):
+        # -O0 for improved debugging, since performance is bound by I/O
         CPU_ARCH = self.cpu_arch()
         SIMD_WIDTH = self.simd_width()
         return [