Project: s41467-024-55419-z | Function: loss_function (python)

system: You are an expert Python programmer implementing algorithms from scientific papers.
Complete the masked function based on the algorithm description.
Follow Python best practices and maintain consistent code style.
Return **only** the complete Python function code.

user: Complete this Python function:

**DESCRIPTION:** Computes the composite loss for StemVAE combining reconstruction loss, KL divergence, and classification loss for time-series single-cell data analysis

**KEY TERMS:** {
  "recons_loss": "reconstruction loss",
  "kld_loss": "KL divergence loss",
  "clf_loss": "classification loss for time labels",
  "kld_weight": "weight balancing reconstruction and KL terms",
  "M_N": "minibatch correction factor"
}

**CONTEXT CODE:**         :return: (Tensor) List of latent codes
        """
        result = self.encoder(input)
        result = torch.flatten(result, start_dim=1)

        # Split the result into mu and var components
        # of the latent Gaussian distribution
        mu = self.fc_mu(result)
        log_var = self.fc_var(result)

        return [mu, log_var]

    def decode(self, z: Tensor) -> Tensor:
        """
        Maps the given latent codes
        onto the image space.
        :param z: (Tensor) [B x D]
        :return: (Tensor) [B x C x H x W]
        """
        result = self.decoder_input(z)  # result shape:(batchSize,64)
        result = self.decoder(result)
        result = self.final_layer(result)
        return result

    def clf_decode(self, z: Tensor) -> Tensor:
        # New: add a classifier
        result = self.clf_input(z)  # result shape:(batchSize,64)
        result = self.clf_decoder(result)
        return result



    def reparameterize(self, mu: Tensor, logvar: Tensor) -> Tensor:
        """
        Reparameterization trick to sample from N(mu, var) from
        N(0,1).
        :param mu: (Tensor) Mean of the latent Gaussian [B x D]
        :param logvar: (Tensor) Standard deviation of the latent Gaussian [B x D]
        :return: (Tensor) [B x D]
        """
        raise NotImplementedError("Function implementation has been masked")

    def forward(self, input: Tensor, **kwargs) -> List[Tensor]:
        labels = kwargs["labels"]
        mu, log_var = self.encode(input)
        z = self.reparameterize(mu, log_var)
        recons = self.decode(z)
        reclf = self.clf_decode(z)
        return [recons, input, mu, log_var, reclf, labels]

    def loss_function(self,
                      *args,
                      **kwargs) -> dict:
        """
        Computes the VAE loss function.
        KL(N(\mu, \sigma), N(0, 1)) = \log \frac{1}{\sigma} + \frac{\sigma^2 + \mu^2}{2} - \frac{1}{2}
        :param args:
        :param kwargs:
        :return:
        """
        raise NotImplementedError("Function implementation has been masked")

    def sample(self,
               num_samples: int,
               current_device: int, **kwargs) -> Tensor:
        """
        Samples from the latent space and return the corresponding
        image space map.
        :param num_samples: (Int) Number of samples
        :param current_device: (Int) Device to run the model
        :return: (Tensor)
        """
        z = torch.randn(num_samples,
                        self.latent_dim)

        z = z.to(current_device)

        samples = self.decode(z)
        return samples

    def generate(self, x: Tensor, **kwargs) -> Tensor:
        """
        Given an input image x, returns the reconstructed image
        :param x: (Tensor) [B x C x H x W]
        :return: (Tensor) [B x C x H x W]
        """

        return self.forward(x)[0]


Complete the function implementation replacing "raise NotImplementedError".