Added "-fopenmp" cflags (#2783) (#3006)

* Fixes #2783
- Added "-fopenmp" cflags
- Refactored cxx flags creation

* Fixed for macosx

setup.py

@@ -175,7 +175,9 @@ 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
@@ -190,16 +192,13 @@ 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')
+    elif sys.platform == 'linux':
+        extra_compile_args['cxx'].append('-fopenmp')
 
     debug_mode = os.getenv('DEBUG', '0') == '1'
     if debug_mode: