Fixup post init (for v5 remot compatibility)

As per title, in v5 `post_init` became mandatory for everything inheriting from pretrained model

modeling_xlm_roberta.py

@@ -480,6 +480,7 @@ class XLMRobertaModel(XLMRobertaPreTrainedModel):
             self.name_or_path, trust_remote_code=True
         )
         self._rotary_emb_base = config.rotary_emb_base
+        self.post_init()
 
     @torch.inference_mode()
     def encode(