Add ability to set predictions filename

[ejm714]

Currently, model predictions will always get written to a file called zamba_predictions.csv (in the specified save_dir).

We may want to add an output_csv_name arg to parameterize this. Relatedly, we should think about how to make predict_configuration.yaml run specific if we think this file will get copied around as well. It may make sense to have a run_name param that gets used for both files.

Fromm: https://github.com/drivendataorg/zamba/pull/151#discussion_r734906935