fix train_new_from_iterator in the case of byte-level tokenizers (#17549)

tests/models/gptj/test_modeling_gptj.py

@@ -155,6 +155,11 @@ class GPTJModelTester:
             rotary_dim=self.rotary_dim,
         )
 
+    def get_pipeline_config(self):
+        config = self.get_config()
+        config.vocab_size = 300
+        return config
+
     def prepare_config_and_inputs_for_decoder(self):
         (
             config,