[Enhance] Add a default value for MMCV_MLU_ARGS (#2688)

* add mlu_args

* add mlu_args

* Modify the code

---------
Co-authored-by: budefei <budefei@cambricon.com>

setup.py

@@ -369,7 +369,7 @@ def get_extensions():
                         exit()
 
             define_macros += [('MMCV_WITH_MLU', None)]
-            mlu_args = os.getenv('MMCV_MLU_ARGS')
+            mlu_args = os.getenv('MMCV_MLU_ARGS', '-DNDEBUG ')
             mluops_includes = []
             mluops_includes.append('-I' +
                                    os.path.abspath('./mlu-ops/bangc-ops'))