refactor(model): opt the migrated families into builder-backed model configs

After #6305, MODEL_CONFIG_CLASS defaults to None and each family opts in.
Set MODEL_CONFIG_CLASS = BridgeGPTModelConfig on the bridges this PR
migrates, matching Qwen3 and Llama, and assert it in their tests.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Signed-off-by: Kamran Jafari <kjafarisadeg@nvidia.com>

Author: kamran-nvidia

src/megatron/bridge/models/qwen/qwen2_bridge.py

@@ -25,6 +25,7 @@
     GatedMLPMapping,
     QKVMapping,
 )
+from megatron.bridge.models.gpt.model_config import BridgeGPTModelConfig
 
 
 @MegatronModelBridge.register_bridge(source=Qwen2ForCausalLM, target=GPTModel, model_type="qwen2")
@@ -45,6 +46,7 @@ class Qwen2Bridge(MegatronModelBridge):
         >>> model_config = bridge.get_model_config()
     """
 
+    MODEL_CONFIG_CLASS = BridgeGPTModelConfig
     USE_MODEL_CONFIG_FOR_CONVERSION = True
 
     def hf_config_to_model_config_kwargs(self, hf_config: PretrainedConfig) -> dict[str, Any]:

tests/unit_tests/models/mimo/test_mimo_bridge.py

@@ -19,6 +19,7 @@
 from transformers import GenerationConfig
 
 from megatron.bridge.models.conversion.model_bridge import MegatronModelBridge, WeightConversionTask
+from megatron.bridge.models.gpt.model_config import BridgeGPTModelConfig
 from megatron.bridge.models.gpt_provider import GPTModelProvider
 from megatron.bridge.models.hf_pretrained.causal_lm import PreTrainedCausalLM
 from megatron.bridge.models.mimo.mimo_bridge import MimoBridge
@@ -132,6 +133,7 @@ def test_provider_bridge_is_inherited_compatibility_only(self):
         assert "provider_bridge" not in MimoBridge.__dict__
 
     def test_conversion_uses_builder_config(self):
+        assert MimoBridge.MODEL_CONFIG_CLASS is BridgeGPTModelConfig
         assert MimoBridge.USE_MODEL_CONFIG_FOR_CONVERSION is True
 
     def test_model_config_maps_mtp_config(self, mock_pretrained_mimo):

tests/unit_tests/models/qwen/test_qwen2_bridge.py

@@ -55,6 +55,7 @@ def test_provider_bridge_is_inherited_compatibility_only(self):
 
     def test_conversion_uses_builder_config(self):
         """Checkpoint conversion constructs Qwen2 through its ModelBuilder."""
+        assert Qwen2Bridge.MODEL_CONFIG_CLASS is BridgeGPTModelConfig
         assert Qwen2Bridge.USE_MODEL_CONFIG_FOR_CONVERSION is True
 
     def test_hf_config_to_model_config_uses_direct_mapping(self):

tests/unit_tests/models/test_gpt_model_config.py

@@ -217,10 +217,12 @@ def test_unmigrated_bridge_keeps_provider_without_warning() -> None:
         vocab_size=256,
     )
 
-    assert Qwen2Bridge.MODEL_CONFIG_CLASS is None
+    class UnmigratedQwen2Bridge(Qwen2Bridge):
+        MODEL_CONFIG_CLASS = None
+
     with warnings.catch_warnings():
         warnings.simplefilter("error", FutureWarning)
-        provider = Qwen2Bridge().provider_bridge(Mock(config=hf_config))
+        provider = UnmigratedQwen2Bridge().provider_bridge(Mock(config=hf_config))
 
     assert provider.position_embedding_type == "rope"
     assert provider.add_qkv_bias is True