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/hy_v3/hy_v3_bridge.py

@@ -27,6 +27,7 @@
     GatedMLPMapping,
     QKVMapping,
 )
+from megatron.bridge.models.gpt.model_config import BridgeGPTModelConfig
 from megatron.bridge.models.gpt_provider import GPTModelProvider
 
 
@@ -61,6 +62,7 @@ class HYV3Bridge(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]:

src/megatron/bridge/models/minimax_m2/minimax_m2_bridge.py

@@ -29,6 +29,7 @@
     MegatronParamMapping,
     QKVMapping,
 )
+from megatron.bridge.models.gpt.model_config import BridgeGPTModelConfig
 from megatron.bridge.models.minimax_m2.minimax_m2_provider import minimax_m2_layer_spec
 
 
@@ -132,6 +133,7 @@ class MiniMaxM2Bridge(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/hy_v3/test_hy_v3_bridge.py

@@ -58,6 +58,7 @@ def test_provider_bridge_is_inherited_compatibility_only(self):
         assert "provider_bridge" not in HYV3Bridge.__dict__
 
     def test_conversion_uses_builder_config(self):
+        assert HYV3Bridge.MODEL_CONFIG_CLASS is BridgeGPTModelConfig
         assert HYV3Bridge.USE_MODEL_CONFIG_FOR_CONVERSION is True
 
     def test_hf_config_to_model_config_uses_direct_mapping(self):

tests/unit_tests/models/minimax_m2/test_minimax_m2_bridges.py

@@ -23,6 +23,7 @@
 from transformers import GenerationConfig
 
 from megatron.bridge.models.conversion.model_bridge import MegatronModelBridge
+from megatron.bridge.models.gpt.model_config import BridgeGPTModelConfig
 from megatron.bridge.models.hf_pretrained.causal_lm import PreTrainedCausalLM
 from megatron.bridge.models.minimax_m2.minimax_m2_bridge import (
     MiniMaxM2Bridge,
@@ -535,6 +536,7 @@ def test_provider_bridge_is_inherited_compatibility_only(self):
         assert "provider_bridge" not in MiniMaxM2Bridge.__dict__
 
     def test_conversion_uses_builder_config(self):
+        assert MiniMaxM2Bridge.MODEL_CONFIG_CLASS is BridgeGPTModelConfig
         assert MiniMaxM2Bridge.USE_MODEL_CONFIG_FOR_CONVERSION is True
 
     def test_hf_config_to_model_config_uses_direct_mapping(self):