[BUGFIX] fix for MXNet 1.5. (#552)

* remove clone.

* turn on numpy compatible.

* Revert "remove clone."

This reverts commit 17bbf76ed72ff178df6b3f35addc428048672457.

python/dgl/backend/mxnet/tensor.py

 from __future__ import absolute_import
 
+from distutils.version import LooseVersion
+
 import numpy as np
 import mxnet as mx
 import mxnet.ndarray as nd
 import numbers
 
+MX_VERSION = LooseVersion(mx.__version__)
+# After MXNet 1.5, empty tensors aren't supprted by default.
+# after we turn on the numpy compatible flag, MXNet supports empty NDArray.
+if MX_VERSION.version[0] == 1 and MX_VERSION.version[1] >= 5:
+    mx.set_np_compat(True)
+
 def data_type_dict():
     return {'float16' : np.float16,
             'float32' : np.float32,