code: def get_prefix_tokens(
      self, batch: Batch
  ) -> Optional[torch.LongTensor]:
      bsz = batch.batchsize
      dev = batch.text_vec.device
      prefix_toks = self.dict.txt2vec(PREFIX_TEXT)
      prefix_toks_batch = [prefix_toks for _ in range(bsz)]
      return torch.LongTensor(prefix_toks_batch).to(dev)
  
imports: import torch
  from typing import Optional
  from parlai.agents.transformer.transformer import TransformerGeneratorAgent
  from parlai.core.torch_agent import Batch
label_desc: Use image node_id 1 for calling the TransformerGeneratorPrefixAgent obj's underlying member method code with example usage: obj.get_prefix_tokens(batch) and returns: torch