Fix: Seq2SeqTrainingArgs overriding to_dict for GenerationConfig json support (#22919)

* Seq2SeqTrainingArgs overriding to_dict for GenerationConfig json support

* seq2seqTrainingArgs to_dict calling super method before handling genconf

src/transformers/training_args_seq2seq.py

@@ -83,3 +83,15 @@ class Seq2SeqTrainingArguments(TrainingArguments):
             "help": "Model id, file path or url pointing to a GenerationConfig json file, to use during prediction."
         },
     )
+
+    def to_dict(self):
+        """
+        Serializes this instance while replace `Enum` by their values and `GenerationConfig` by dictionaries (for JSON
+        serialization support). It obfuscates the token values by removing their value.
+        """
+        # filter out fields that are defined as field(init=False)
+        d = super().to_dict()
+        for k, v in d.items():
+            if isinstance(v, GenerationConfig):
+                d[k] = v.to_dict()
+        return d