[Feature] Add environment variable to switch on/off MXNet set_np_shape (#1207)

* add env var

* Trigger CI

* simplification

* add doc

docs/source/install/backend.rst

@@ -21,6 +21,11 @@ MXNet uses uint32 as the default data type for integer tensors, which only suppo
 size smaller than 2^32. To enable large graph training, *build* MXNet with ``USE_INT64_TENSOR_SIZE=1``
 flag. See `this FAQ <https://mxnet.apache.org/api/faq/large_tensor_support>`_ for more information.
 
+MXNet 1.5 and later has an option to enable Numpy shape mode for ``NDArray`` objects, some DGL models
+need this mode to be enabled to run correctly. However, this mode may not compatible with pretrained 
+model parameters with this mode disabled, e.g. pretrained models from GluonCV and GluonNLP.
+By setting ``DGL_MXNET_SET_NP_SHAPE``, users can switch this mode on or off.
+
 Tensorflow backend
 ------------------
 

python/dgl/backend/mxnet/tensor.py

@@ -2,6 +2,7 @@ from __future__ import absolute_import
 
 from distutils.version import LooseVersion
 
+import os
 import numpy as np
 import mxnet as mx
 import mxnet.ndarray as nd
@@ -17,7 +18,7 @@ if MX_VERSION.version[0] == 1 and MX_VERSION.version[1] < 5:
 
 # After MXNet 1.5, empty tensors aren't supprted by default.
 # After we turn on the numpy compatible flag, MXNet supports empty NDArray.
-mx.set_np_shape(True)
+mx.set_np_shape(bool(os.environ.get('DGL_MXNET_SET_NP_SHAPE', True)))
 
 def data_type_dict():
     return {'float16' : np.float16,