✨ 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/qwen3.py

@@ -187,7 +187,7 @@ class Qwen3Model(nn.Module):
         return hidden_states
 
 
-@register_model("Qwen3ForCausalLM", "Qwen2ForCausalLM")
+@register_model("Qwen3ForCausalLM")
 class Qwen3ForCausalLM(nn.Module):
     packed_modules_mapping = {
         "q_proj": ("qkv_proj", "q"),