Fix seq pack flag in get_logprobs

[mathemakitten]

What does this PR do ?

#3167 / #2966 added a forward pass which does not gate correctly on whether sequence packing is enabled.

Contribution process

flowchart LR
    A[Pre-checks] --> B[PR Tests]
    subgraph Code Review/Approval
        C1[Expert Review] --> C2[Final Review]
    end
    B --> C1
    C2 --> D[Merge]

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[yobibyte]

Thanks for the fix, sorry for breaking you!