fix the doc building (#268)

.readthedocs.yml

+version: 2
+
+python:
+  version: 3.7
+  install:
+    - requirements: docs/requirements.txt

docs/conf.py

@@ -48,9 +48,7 @@ extensions = [
     'recommonmark',
 ]
 
-autodoc_mock_imports = [
-    'cv2', 'enum', 'pathlib', 'mmcv._ext', 'pytorch', 'torchvision'
-]
+autodoc_mock_imports = ['cv2', 'mmcv._ext', 'torchvision']
 
 # Add any paths that contain templates here, relative to this directory.
 templates_path = ['_templates']

docs/requirements.txt

+torch