W&B integration improvements
Hey guys,

I'm one of the founders of W&B and have been working with @borisdayma on the wandb integration.  I made a few changes to address a few cases we thought through:

1. If wandb is installed but the user hasn't logged in we now print a warning instead of prompting for an api_key
2. Made messaging more clear on how to install wandb and linked to more documentation
3. Added an environment variable for disabling wandb.  This is important for users using fastai or lightning with their own wandb callback.
4. Added an environment variable for disabling wandb.watch.  Because we're pulling gradients and parameters during training there is a small performance impact on training.  We wanted to make it easy for users to disable this feature.

Happy to iterate on any feedback you guys have.  Really excited about this integration!
^^ I borked your PR unfortunately (💩). Sorry about that. Care to open a new one?