Balancer for different model outputs

[gmongaras]

Added balancer support for different outputs from the model.
For example, if a model outputted two terms, x1 and x2, and the losses are:
losses['loss_a'] = compute_loss_a(x1, y1)
losses['loss_b'] = compute_loss_b(x2, y2)

Then the balancer needs to work with both x1 and x2, not just x. This addition adds this change, while keeping support for a single output as well.

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