Experiment with calling dataset.load() _after_ joining examples into batch

[JackKelly]

For the Zarr DataSources, it may be faster to load the data into memory after joining (lazily loaded) examples.

i.e. call .load() towards the end of get_batch() instead of at the end of get_example().

This should allow dask to do a better job of scheduling what needs to be done. And might result in faster times per batch.