get_weights_report.py: load dataset from file if its not built in

As discussed in #1010, there seems to be a bug in `get_weights_report.py` that prevents the completion of the quick start example, and I believe it's due to the difference between built in and imported datasets. This patch checks for builtin datasets using a string comparison, and loads the file if necessary. There might be a proper method to do this check, but I'm just familiarizing myself with the codebase so I used an ad hoc test. It worked for several tests under in the tutorials, including the offending CIFAR dataset in the quick start example.