Add instance types to benchmark notebook

[jacobtomlinson]

In the HPO benchmarking notebook added in #249 I noticed that the VM types used for testing aren't mentioned.

For instance, we find a 48x speedup in wall clock time (0.71 hrs vs 34.6 hrs) for XGBoost and 16x (3.86 hrs vs 63.2 hrs) for RandomForest when comparing between GPU and CPU EC2 instances on 100 HPO trials on 3 years of the Airline Dataset.

Could you update this to say which VM types were used (and which GPUs)?