Allow user to change interval at which validation loss is checked

[njbernstein]

Currently the scVI model is trained with the following invocation in which check_val_every_n_epoch argument is hardcoded at 5:

        vae.train(
            max_epochs=2000,
            validation_size=valid_pct,
            check_val_every_n_epoch=5,
            plan_kwargs=plan_kwargs,
            callbacks=scvi_callbacks,
        )

We'd like to allow users to specify this value using the Solo command line tool.

There are two ways this can be addressed via a command line argument or the model_json_file which specifies model parameters. In this case we will use model_json_file to allow users to change this argument for training.

The example json file can be seen here: https://github.com/calico/solo/blob/master/solo_params_example.json