fix doc building error

docs/conf.py

@@ -12,11 +12,10 @@
 # add these directories to sys.path here. If the directory is relative to the
 # documentation root, use os.path.abspath to make it absolute, like shown here.
 #
-import os
-import sys
-sys.path.insert(0, os.path.abspath('..'))
-
-import mmcv  # noqa: E402
+version_file = '../mmcv/version.py'
+with open(version_file, 'r') as f:
+    exec(compile(f.read(), version_file, 'exec'))
+__version__ = locals()['__version__']
 
 # -- Project information -----------------------------------------------------
 
@@ -25,9 +24,9 @@ copyright = '2018, Kai Chen'
 author = 'Kai Chen'
 
 # The short X.Y version
-version = mmcv.__version__
+version = __version__
 # The full version, including alpha/beta/rc tags
-release = mmcv.__version__
+release = __version__
 
 # -- General configuration ---------------------------------------------------