import torch
from torch import nn
from transformers import PreTrainedModel

class CustomClassifier(PreTrainedModel):
    config_class = CustomClassificationConfig

    def __init__(self, config):
        super().__init__(config)
        self.encoder = nn.Sequential(
            nn.Linear(config.input_dim, config.hidden_dim),
            nn.ReLU(),
            nn.Linear(config.hidden_dim, config.hidden_dim),
            nn.ReLU(),
        )
        self.classifier = nn.Linear(config.hidden_dim, config.num_classes)

    def forward(self, input_ids=None, labels=None, **kwargs):
        # input_ids: shape (batch_size, input_dim)
        hidden = self.encoder(input_ids)
        logits = self.classifier(hidden)

        loss = None
        if labels is not None:
            loss_fn = nn.CrossEntropyLoss()
            loss = loss_fn(logits, labels)

        return {"loss": loss, "logits": logits}