Remove LocalBackend export from root __init__.py

[arcticfly]

On non-linux clients, importing ART currently causes a ModuleNotFoundError: No module named 'torch' error because LocalBackend requires torch. Looks like LocalBackend was added to art/init.py a couple weeks ago, but we don't need it to be there. Users can instead import LocalBackend from art.local.

Changes

• Remove LocalBackend export from art/init.py
• Allow user to pass in arbitrary logged run name to log_constant_metrics_wandb

[corbt]

@arcticfly are there examples that will need to be updated?

[arcticfly]

are there examples that will need to be updated?

Not unless you added one in the last two weeks, which is when we started exporting LocalBackend from the root.