70_callback.wandb: Create Docments

[kevinbird15]

Add docments to 70_callback.wandb.

See the style guide and contribution guide for more details (links will be added when created).

A solid understanding of how callbacks work and weights & biases would definitely be a plus here

Docments progress:

• WandbCallback
• log_dataset
• log_model
• wandb_process (maybe, typedispatch might trip you up here)

[dienhoa]

Hi @tmabraham. Can I work with you on this issue? Not considering myself having a very solid understanding of fast.ai callback and very little knowledge of WandB. Actually, I want to learn WandB and thinking docment WandB fastai's callback would be a great opportunity.

Thanks