Make wandb checkpoint logging compatible with wandb model registry

[growlix]

What does this PR do?

This PR modifies wandb_logger.py to instantiate model checkpoints as type "model" instead of ".pt", to allow compatibility with W&B's new model registry feature, which MosaicML and WandB are cross-promoting with a demo/webinar next week.

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[eracah]

LGTM. Just curious: do you have a link to a wandb run that used this?

[growlix]

@eracah

LGTM. Just curious: do you have a link to a wandb run that used this?
Why yes I do