Support offline SpeechLM2 HF exports

[pzelasko]

What changed

This is an alternative implementation for the offline-loading goal in #15695.

It keeps the root checkpoint config.json as the NeMo/SpeechLM wrapper config and saves the original LLM HF config under llm_backbone/config.json to avoid a config.json filename conflict. The tokenizer remains saved once at the checkpoint root for vLLM compatibility.

At load time, HFHubMixin detects the local export layout and redirects:

• root tokenizer files -> cfg["tokenizer_path"]
• llm_backbone/config.json -> cfg["pretrained_llm"] or cfg["pretrained_lm_name"]

This keeps old checkpoints backward compatible because configs without these local files are left unchanged.

Compatibility coverage

• SALM
• SALMAutomodel
• SALMWithAsrDecoder
• Duplex S2S
• Duplex S2S speech decoder
• Duplex STT
• Duplex EAR-TTS

Testing

• python -m py_compile examples/speechlm2/to_hf.py nemo/collections/speechlm2/parts/hf_hub.py tests/collections/speechlm2/test_hf_hub.py tests/collections/speechlm2/test_to_hf.py
• python -m black --check examples/speechlm2/to_hf.py nemo/collections/speechlm2/parts/hf_hub.py tests/collections/speechlm2/test_to_hf.py tests/collections/speechlm2/test_hf_hub.py
• python -m isort --check-only examples/speechlm2/to_hf.py nemo/collections/speechlm2/parts/hf_hub.py tests/collections/speechlm2/test_to_hf.py tests/collections/speechlm2/test_hf_hub.py
• pytest tests/collections/speechlm2/test_to_hf.py tests/collections/speechlm2/test_hf_hub.py -q

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[pzelasko]

/ok to test f33caf8

[pzelasko]

/ok to test a8d8276

[github-actions]

[🤖]: Hi @pzelasko 👋,

We wanted to let you know that a CICD pipeline for this PR just finished successfully.

So it might be time to merge this PR or get some approvals.