Fix loss masking

[oleksost]

✨ Description

Addresses #442

• loss_masks should include padding and image placeholder tokens

TODO:

• not sure if loss masking spans and image placeholders should also be ignored in the layer-wise losses

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[jlamypoirier]

Is this really what we want? We can't train the model to produce these labels, but it might make sense to compute other losses?

[jlamypoirier]

Can we skip this when not needed?

[jlamypoirier]

Activation distillation ignores loss_mask, it uses activation_mask instead.

[jlamypoirier]

TODO:

• not sure if loss masking spans and image placeholders should also be ignored in the layer-wise losses

Does that even make sense? These refer to token prediction which isn't really a thing at the activation stage. I guess we could take the next token but that raises several concerns (especially with MTP).

Actually I think we shouldn't mask those. They may not be used for next token prediction, but the keys and values resulting from these activations are used in further down in the sequence, which means we do train these activations.