[Tunix] Support scanned to unscanned weight transfer in transfer_state_directly

[gagika]

This PR extends the transfer_state_directly utility to support weight synchronization from scanned MaxText model (where layers are stacked in a single tensor) to unscanned MaxText + vLLM models (where layers are separate parameters).

Previously, transfer_state_directly only supported 1-to-1 mapping (Unscanned -> Unscanned). This change adds logic to detect and unroll scanned layers during the transfer process.

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