Fix: don't enter branch if mtp_num_layers == 0

[rj42]

The condition if self.config.mtp_num_layers is not None evaluates to True when mtp_num_layers=0, causing the code to enter the MTP branch and crash on labels.clone() when labels=None.

Since range(0) produces an empty loop and torch.chunk(..., 1) is essentially an identity operation, entering this branch with mtp_num_layers=0 does nothing useful but requires labels to be passed.

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[yaox12]

Good catch. Thanks! Can you open a PR to the main branch as well?

[yaox12]

/ok to test 6e5fada

[Victarry]

/ok to test 41c953d

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