fix pad token id issue

vit_gpt2/configuration_vit_gpt2.py

@@ -31,6 +31,13 @@ class ViTGPT2Config(PretrainedConfig):
 
         self.is_encoder_decoder = True
 
+        # Required in `generate()`.
+        self.bos_token_id = self.text_config.bos_token_id
+        self.eos_token_id = self.text_config.eos_token_id
+
+        assert hasattr(self.text_config, 'pad_token_id')
+        self.pad_token_id = self.text_config.pad_token_id
+
         self.decoder_start_token_id = self.text_config.bos_token_id
         self.forced_eos_token_id = self.text_config.eos_token_id