✨ feat: add Qwen2/2.5 model support

Separate Qwen2 from Qwen3 implementation:
- Qwen2: Uses QKV bias, no QK norm
- Qwen3: Has optional QK norm when no bias

Tested with Qwen2.5-7B-Instruct-1M, RULER niah_single_1 passed.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

nanovllm/models/__init__.py

@@ -3,6 +3,7 @@
 from nanovllm.models.registry import register_model, get_model_class, MODEL_REGISTRY
 
 # Import models to trigger registration
+from nanovllm.models import qwen2
 from nanovllm.models import qwen3
 from nanovllm.models import llama
 from nanovllm.models import glm4