Cherry-pick #3399 for Mamba Uneven PP fix

[kevalmorabia97]

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Cherry-pick #3399

Mamba implementation does not consider uneven PP and always splits layers evenly across all PP ranks resulting in silently dropping some layers!

Tested PR with Nemotron3-nano-30B (52 layers) on 8-gpu (5-7-7-7-7-7-7-5 per rank)

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flowchart LR
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    subgraph Code Review/Approval
        C1[Expert Review] --> C2[Final Review]
    end
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[janEbert]

/ok to test 62ac0e8