Fix packed seq shift when using answer only loss

[cuichenx]

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What does this PR do ?

Fix incorrect shift when using answer only loss

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[cuichenx]

linting issues are with the nemo 1 files which will be deprecated soon. The equivalent nemo 2 file have these issues already fixed

[ryanleary]

If we verify that this resolves the convergence issue, can we please add some unit testing around the loss mask generation? Ideally we'd add that testing as well as some short convergence test around the recipe.

[cuichenx]

If we verify that this resolves the convergence issue, can we please add some unit testing around the loss mask generation? Ideally we'd add that testing as well as some short convergence test around the recipe.

good idea! I can do that

[github-actions]

[🤖]: Hi @cuichenx 👋,

We wanted to let you know that a CICD pipeline for this PR just finished successfully

So it might be time to merge this PR or get some approvals

I'm just a bot so I'll leave it you what to do next.

//cc @pablo-garay @ko3n1g