[Enhance] Add a default value for MMCV_MLU_ARGS (open-mmlab#2686)

* add mlu_args

* Update setup.py

Co-authored-by: Zaida Zhou <58739961+zhouzaida@users.noreply.github.com>

---------

Co-authored-by: budefei <budefei@cambricon.com>
Co-authored-by: Zaida Zhou <58739961+zhouzaida@users.noreply.github.com>

setup.py

@@ -368,7 +368,7 @@ def get_mluops_version(file_path):
                         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'))