gpu training example fix #88

Job	Run time
Publish Image (lightgbm-dist-py-test, examples/xgboost/lightgbm-dist/Dockerfile, examples/xgboost... / Publish Image	3m 18s
Publish Image (mxnet-auto-tuning, examples/mxnet/tune/Dockerfile, examples/mxnet/tune) / Publish Image	2m 19s
Publish Image (mxnet-gpu, examples/mxnet/train/Dockerfile) / Publish Image	1m 40s
Publish Image (pytorch-dist-sendrecv-test, examples/pytorch/smoke-dist/Dockerfile) / Publish Image	1m 10s
Publish Image (pytorch-elastic-example-echo, examples/pytorch/elastic/echo/Dockerfile) / Publish Image	5m 20s
Publish Image (pytorch-elastic-example-imagenet, examples/pytorch/elastic/imagenet/Dockerfile) / Publish Image	3m 13s
Publish Image (tf-dist-mnist-test, examples/tensorflow/dist-mnist/Dockerfile) / Publish Image	2m 1s
Publish Image (tf-distributed-worker, examples/tensorflow/distribution_strategy/estimator-API/Doc... / Publish Image	1m 37s
Publish Image (tf-mnist-with-summaries, examples/tensorflow/mnist_with_summaries/Dockerfile) / Publish Image	1m 39s
Publish Image (tf-multi-worker-strategy, examples/tensorflow/distribution_strategy/keras-API/Dock... / Publish Image	3m 23s
Publish Image (tf-smoke, examples/tensorflow/tf_sample/Dockerfile) / Publish Image	48s
Publish Image (xgboost-dist-iris, examples/xgboost/xgboost-dist/Dockerfile, examples/xgboost/xgbo... / Publish Image	1m 47s
Publish Image (xgboost-dist-rabit-test, examples/xgboost/smoke-dist/Dockerfile, examples/xgboost/... / Publish Image	1m 30s
	29m 45s

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