fix mamba2 ssd test for varlen refactor

[RishiAstra]

Essential Elements of an Effective PR Description Checklist

• The purpose of the PR, such as "Fix some issue (link existing issues this PR will resolve)".
• The test plan, such as providing test command.
• The test results, such as pasting the results comparison before and after, or e2e results
• (Optional) The necessary documentation update, such as updating supported_models.md and examples for a new model.

Purpose

Fix Mamba2 SSD test to work with new varlen refactor

Test Plan

This is a test

Test Result

Test passes the first few, didn't fully test, but it's no longer failing asserts

(Optional) Documentation Update

[github-actions]

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[cyang49]

/lgtm