allow checkpointing setup to be customized

[camallen]

add params for checkpoint setup to the train_default_zoobot_from_scratch function. specifically this PR provides params to change the

• checkpoint name with / without the metric names
• last_k outputs

e.g.

train_with_pytorch_lightning.train_default_zoobot_from_scratch(
....
checkpoint_file_template='epoch-{epoch}-supervised_loss-{val/supervised_loss:.2f}',
auto_insert_metric_name=False,
save_top_k=1
)

[mwalmsley]

Hi Cam. Another nice feature, thanks! I see that filename=None gives the default name so I assume this will not break any existing setups. Approved.