[fsdp] fix: Handle different transformers versions for Vision2Seq models in FSDP model merger (verl-project#5108)

### What does this PR do?

Transformers versions 4.54 and later use the AutoModelForImageTextToText
class to load multimodal models, therefore a condition should be added
to the model merger in FSDP.

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Co-authored-by: zhangliangxu3 <zhangliangxu3@jd.com>

Author: liangxuZhang

verl/model_merger/base_model_merger.py

@@ -24,7 +24,6 @@
     AutoConfig,
     AutoModelForCausalLM,
     AutoModelForTokenClassification,
-    AutoModelForVision2Seq,
     GenerationConfig,
 )
 
@@ -202,7 +201,20 @@ def get_transformers_auto_model_class(self):
                 case "AutoModelForTokenClassification":
                     return AutoModelForTokenClassification
                 case "AutoModelForVision2Seq":
-                    return AutoModelForVision2Seq
+                    # Handle different transformers versions for Vision2Seq models
+                    import transformers
+                    from packaging import version
+
+                    if version.parse(transformers.__version__) >= version.parse("4.54.0"):
+                        # transformers >= 4.54.0 uses AutoModelForImageTextToText
+                        from transformers import AutoModelForImageTextToText
+
+                        return AutoModelForImageTextToText
+                    else:
+                        # transformers < 4.54.0 uses AutoModelForVision2Seq
+                        from transformers import AutoModelForVision2Seq
+
+                        return AutoModelForVision2Seq
                 case _:
                     raise NotImplementedError(f"Unknown auto class {auto_class}")
         else: