docs(models.rst): explain how to use wandb

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DeepChem extensively uses `Keras`_ to build powerful machine learning models.

Training loss and validation metrics can be automatically logged to `Weights & Biases`_ with the following commands::

  # Install wandb in shell

  pip install wandb

  # Login in shell (required only once)

  wandb login

  # Start a W&B run in your script (refer to docs for optional parameters)

  wandb.init(project="my project")

  # Set `wandb` arg when creating `KerasModel`

  model = KerasModel(…, wandb=True)

.. _`Keras`: https://keras.io/

.. _`Weights & Biases`: http://docs.wandb.com/

.. autoclass:: deepchem.models.KerasModel

  :members: