[MXNET] mxnet_mnist.ipynb issues

https://github.com/awslabs/amazon-sagemaker-examples/blob/master/sagemaker-python-sdk/mxnet_mnist/mxnet_mnist.ipynb

Most of the notebook runs; 3/4 of it without any errors:
The only I get near the end of the notebook is the following:
`UnexpectedStatusException: Error for Compilation job compilation-mxnet-training-2020-08-12-20-56-31-867: Failed. Reason: ClientError: InputConfiguration: S3 object does not exist. Bucket: sagemaker-us-east-1-688520471316, Key: mxnet-mnist-example/artifacts/mxnet-training-2020-08-12-20-52-18-787/output/model.tar.gz`

Open sagemaker-python-sdk/mxnet_mnist/mxnet_mnist.ipynb and run the notebook through the compilation step near the end. Check the reported missing S3 object, mxnet-training-2020-08-12-20-52-18-787/output/model.tar.gz, and trace where that artifact is expected to be produced. Done means the notebook completes this step without the reported missing-object error.

Written by the indexing model from the issue text.