code: def test_roberta_forward_backward(self, device: str):
      _, model = get_toy_model(device)
      sample = mk_sample("en", device)
      en_tokens = sample["net_input"]["src_tokens"]
      (bs, l) = en_tokens.shape
      # Forward
      logits, _ = model(**sample["net_input"])
      self.assertEqual(logits.shape, (bs, l, VOCAB_SIZE))
  
      # Backward
      loss = logits.sum()
      loss.backward()
  
imports: import functools
  import unittest
  from typing import Any, Dict, Sequence
  import fairseq
  import fairseq.options
  import fairseq.tasks
  import torch
  from tests.utils import dummy_dictionary
label_desc: Use image node_id 6 for calling the RobertaTest obj's underlying member method code with example usage: obj.test_roberta_forward_backward(device) without return types