Notebooks are failing to download mnist data from deeplearning.net

deeplearning.net is down. We can download the data from S3 instead - https://github.com/aws/amazon-sagemaker-examples/pull/1770

Here are the notebooks need to be fixed - https://github.com/aws/amazon-sagemaker-examples/search?q=deeplearning

Search the repository for “deeplearning” to identify the affected notebooks, then compare the proposed S3 approach in pull request 1770. Update each MNIST download reference to use S3 and verify that the affected notebooks can obtain the data without accessing deeplearning.net.

Written by the indexing model from the issue text.