set empty cxx flags as default (#3474)

Co-authored-by: Philip Meier <pmeier.github@posteo.de>
Co-authored-by: Francisco Massa <fvsmassa@gmail.com>

setup.py

@@ -181,7 +181,7 @@ def get_extensions():
 
     define_macros = []
 
-    extra_compile_args = {}
+    extra_compile_args = {'cxx': []}
     if (torch.cuda.is_available() and ((CUDA_HOME is not None) or is_rocm_pytorch)) \
             or os.getenv('FORCE_CUDA', '0') == '1':
         extension = CUDAExtension
@@ -196,15 +196,11 @@ def get_extensions():
         else:
             define_macros += [('WITH_HIP', None)]
             nvcc_flags = []
-        extra_compile_args = {
-            'cxx': [],
-            'nvcc': nvcc_flags,
-        }
+        extra_compile_args["nvcc"] = nvcc_flags
 
     if sys.platform == 'win32':
         define_macros += [('torchvision_EXPORTS', None)]
 
-        extra_compile_args.setdefault('cxx', [])
         extra_compile_args['cxx'].append('/MP')
 
     debug_mode = os.getenv('DEBUG', '0') == '1'