Warn if torchvision imported from repo root (#2759)

* Warn if torchvision imported from repo root

This is likely to fail as mentioned in #2239

* Only warn if ops are missing

torchvision/__init__.py

 import warnings
+import os
 
 from .extension import _HAS_OPS
 
@@ -16,6 +17,14 @@ try:
 except ImportError:
     pass
 
+# Check if torchvision is being imported within the root folder
+if (not _HAS_OPS and os.path.dirname(os.path.realpath(__file__)) ==
+        os.path.join(os.path.realpath(os.getcwd()), 'torchvision')):
+    message = ('You are importing torchvision within its own root folder ({}). '
+               'This is not expected to work and may give errors. Please exit the '
+               'torchvision project source and relaunch your python interpreter.')
+    warnings.warn(message.format(os.getcwd()))
+
 _image_backend = 'PIL'
 
 _video_backend = "pyav"