feat: add --num_sanity_val_steps CLI argument to train_model

[Kirwa-web]

Describe your changes

Adds a --num_sanity_val_steps argument to control the number of sanity validation steps run before training, as referenced in #138.

• Adds --num_sanity_val_steps to the Training Options argument group (default: 2, matching PyTorch Lightning's default)
• Passes the argument to pl.Trainer via num_sanity_val_steps=args.num_sanity_val_steps

This is useful for debugging training runs without waiting through full sanity validation steps.

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[Kirwa-web]

@joeloskarsson would appreciate a review when you get a chance. This adds the --num_sanity_val_steps argument referenced in #138.

[joeloskarsson]

Thanks for the addition, this looks good to me!

I don't see a reasonable way to add a test for this, as that would pretty much mean testing pure pytorch lightning functionality. We are here just exposing an argument of the pl.Trainer.