Setup: Revision - Revise torch extra_require (#177)

**Description**
change the minimal version requirement for superbench:
```
'torch>=1.7.0a0',
'torchvision>=0.8.0a0',
```

setup.py

@@ -159,8 +159,8 @@ def run(self):
             'yapf>=0.30.0',
         ],
         'torch': [
-            'torch>=1.7.0',
-            'torchvision>=0.8.0',
+            'torch>=1.7.0a0',
+            'torchvision>=0.8.0a0',
             'transformers>=4.3.3',
         ],
         'nvidia': ['py3nvml>=0.2.6']