feat(qwen3.5+trainer): MTP training loss plumbing aligned with #46229

[curnane-lab]

Wire up Multi-Token Prediction (MTP) training for Qwen3.5 by reusing the inference-side MtpLayer from huggingface#46229 instead of the inline MTP definition from huggingface#45638, and add the matching Trainer hook that combines the auxiliary loss.

Model side (Qwen3.5):

• CausalLMOutputWithPast gains an mtp_loss field carrying the unweighted MTP loss (averaged over MTP layers and non-ignored tokens).
• Qwen3_5TextConfig exposes num_nextn_predict_layers (canonical MTP layer count, aligned with [generate] Add proper MTP support huggingface/transformers#46229) and output_mtp_loss (training-only switch).
• Qwen3_5ForCausalLM dynamically instantiates MtpLayer copies that mirror the main decoder layer architecture (full / linear attention mix, RMSNorm variant) and recomputes the per-layer loss with properly sliced inputs, position embeddings, and labels.

Trainer side:

• TrainingArguments gains mtp_loss_coef (default 0.0, BC-safe).
• Trainer.compute_loss adds the unweighted output.mtp_loss * mtp_loss_coef to the main loss at the very end, leaving the model forward signature clean and the combination policy centralized in the trainer.

Supersedes huggingface#45638's inline-MTP approach in favor of a single shared MtpLayer implementation that the inference path already uses.

Refs: huggingface#46229, huggingface#45638

What does this PR do?

Fixes # (issue)

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