```python
class GPTRewardModel(nn.Module):
    def __init__(self):
        super().__init__()
        model = AutoModelForCausalLM.from_pretrained("pvduy/vicuna-13b-v1.1")
        self.config = model.config
        self.config.n_embd = self.config.hidden_size if hasattr(self.config, "hidden_size") else self.config.n_embd
        self.transformer = model.model
        self.v_head = nn.Linear(self.config.n_embd, 1, bias=False)
        self.tokenizer = AutoTokenizer.from_pretrained("pvduy/vicuna-13b-v1.1")
        self.tokenizer.pad_token_id = self.tokenizer.eos_token_id
        self.tokenizer.padding_side = "right"
        self.PAD_ID = self.tokenizer.pad_token_id


    def forward(
        self,
        input_ids=None,
        past_key_values=None,
        attention_mask=None,
        token_type_ids=None,
        position_ids=None,
        head_mask=None,
        inputs_embeds=None,
        mc_token_ids=None,
        labels=None,
        return_dict=False,
        output_attentions=False,
        output_hidden_states=False,
    ):
        loss = None
        transformer_outputs = self.transformer(
            input_ids,
            attention_mask=attention_mask,
        )
        hidden_states = transformer_outputs[0]

        rewards = self.v_head(hidden_states).squeeze(-1)
        ends = torch.argmax((input_ids == self.PAD_ID).type(torch.float32), dim=1).view(-1, 1)
        rewards = torch.gather(rewards, 1, ends)
        return rewards

```