fix horovod import in rtd

docs/conf.py

@@ -30,7 +30,7 @@ MOCK_MODULES = ['scipy', 'tabulate',
                 'cv2', 'scipy.io', 'dill', 'zmq', 'subprocess32', 'lmdb',
                 'tornado.concurrent', 'tornado',
                 'msgpack', 'msgpack_numpy',
-                'gym', 'functools32', 'horovod.tensorflow']
+                'gym', 'functools32', 'horovod', 'horovod.tensorflow']
 for mod_name in MOCK_MODULES:
     sys.modules[mod_name] = mock.Mock(name=mod_name)
 sys.modules['cv2'].__version__ = '3.2.1'    # fake version

tensorpack/train/trainers.py

@@ -268,5 +268,8 @@ class HorovodTrainer(SingleCostTrainer):
 from ..utils.develop import create_dummy_class   # noqa
 try:
     import horovod.tensorflow as hvd
+except ImportError:
+    HorovodTrainer = create_dummy_class('HovorodTrainer', 'horovod')    # noqa
 except Exception:      # could be other than ImportError, e.g. NCCL not found
+    print("Horovod is installed but cannot be imported.")
     HorovodTrainer = create_dummy_class('HovorodTrainer', 'horovod')    # noqa