Fix ValueError for unsupported LLM architectures in InternVLChatConfig

[Mr-Neutr0n]

Summary

• Use AutoConfig.for_model as a fallback when the LLM architecture is not explicitly listed in InternVLChatConfig, instead of raising a ValueError
• This allows InternVL to work with newer LLM backends like Qwen3ForCausalLM without requiring code changes for each new architecture
• Fix the default llm_config when None is passed: previously set {'architectures': ['']} which produced a misleading ValueError: Unsupported architecture: with an empty string

Fixes #1203

Test plan

• Verify loading InternVL models with Qwen3ForCausalLM backend works without error
• Verify existing supported architectures (Llama, InternLM2, Phi3, Qwen2) still use their specific config classes
• Verify llm_config=None falls back to LlamaConfig without raising

[Mr-Neutr0n]

Friendly bump! Let me know if there's anything I should update or improve to help move this forward.