Deprecate NNVM warning msg (#4333)

nnvm/python/nnvm/__init__.py

@@ -2,6 +2,7 @@
 # coding: utf-8
 """NNVM python API for ease of use and help new framework establish python API. """
 from __future__ import absolute_import as _abs
+import warnings
 
 from . import _base
 from . import symbol as sym
@@ -10,3 +11,6 @@ from ._base import NNVMError
 from . import frontend
 
 __version__ = _base.__version__
+
+warnings.warn("NNVM is deprecated and will be removed in a future version. Use Relay instead.",
+              FutureWarning)