Fix torchvision install

[yf225]

Trying to fix the following error:

May 23 16:08:11 Traceback (most recent call last):
May 23 16:08:11   File "/opt/conda/lib/python3.6/site-packages/sphinx_gallery/gen_rst.py", line 394, in _memory_usage
May 23 16:08:11     out = func()
May 23 16:08:11   File "/opt/conda/lib/python3.6/site-packages/sphinx_gallery/gen_rst.py", line 382, in __call__
May 23 16:08:11     exec(self.code, self.globals)
May 23 16:08:11   File "/var/lib/jenkins/workspace/beginner_source/finetuning_torchvision_models_tutorial.py", line 49, in <module>
May 23 16:08:11     import torchvision
May 23 16:08:11   File "/opt/conda/lib/python3.6/site-packages/torchvision/__init__.py", line 1, in <module>
May 23 16:08:11     from torchvision import models
May 23 16:08:11   File "/opt/conda/lib/python3.6/site-packages/torchvision/models/__init__.py", line 11, in <module>
May 23 16:08:11     from . import detection
May 23 16:08:11   File "/opt/conda/lib/python3.6/site-packages/torchvision/models/detection/__init__.py", line 1, in <module>
May 23 16:08:11     from .faster_rcnn import *
May 23 16:08:11   File "/opt/conda/lib/python3.6/site-packages/torchvision/models/detection/faster_rcnn.py", line 7, in <module>
May 23 16:08:11     from torchvision.ops import misc as misc_nn_ops
May 23 16:08:11   File "/opt/conda/lib/python3.6/site-packages/torchvision/ops/__init__.py", line 1, in <module>
May 23 16:08:11     from .boxes import nms, box_iou
May 23 16:08:11   File "/opt/conda/lib/python3.6/site-packages/torchvision/ops/boxes.py", line 2, in <module>
May 23 16:08:11     from torchvision import _C
May 23 16:08:11 ImportError: /opt/conda/lib/python3.6/site-packages/torchvision/_C.cpython-36m-x86_64-linux-gnu.so: undefined symbol: _ZN2at19NonVariableTypeMode10is_enabledEv

by using the stock version of torchvision.

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