Define API for accessing DataLoader class attributes

[aribrill]

All HDF5DataLoader class attributes are currently specific to that class as opposed to DataLoader, but are accessed outside the class in run_model.py.

When defining data_loader, generator_output_dtypes, map_fn_output_dtypes, output_names, and output_is_label are all accessed. It doesn't seem these are specific to the data format, so if they are made part of the DataLoader base class, then this entire section except for data_loader = HDF5DataLoader() can be pulled out of the if clause, resulting in cleaner code.

Similarly, when getting the event indices in predict mode, example_type and examples are accessed and the event index names are manually defined, but again it may be possible to define all these in such a way that the event indices whatever they may be could be accessed for any DataLoader without relying on specific attributes for each DataLoader subclass.

This issue is probably best tackled at the time we actually add another DataLoader subclass.