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Research projects tend to test different approaches to the same dataset. This is very easy to do in Lightning with inheritance.

For example, imagine we now want to train an Autoencoder to use as a feature extractor for MNIST images. Recall that LitMNIST already defines all the dataloading etc… The only things that change in the Autoencoder model are the init, forward, training, validation and test step.

class Encoder(torch.nn.Module):
    pass

class Decoder(torch.nn.Module):
    pass

class AutoEncoder(LitMNIST):

    def __init__(self):
        super().__init__()
        self.encoder = Encoder()
        self.decoder = Decoder()

    def forward(self, x):
        generated = self.decoder(x)

    def training_step(self, batch, batch_idx):
        x, _ = batch

        representation = self.encoder(x)
        x_hat = self(representation)

        loss = MSE(x, x_hat)
        return loss

    def validation_step(self, batch, batch_idx):
        return self._shared_eval(batch, batch_idx, 'val')

    def test_step(self, batch, batch_idx):
        return self._shared_eval(batch, batch_idx, 'test')

    def _shared_eval(self, batch, batch_idx, prefix):
        x, y = batch
        representation = self.encoder(x)
        x_hat = self(representation)

        loss = F.nll_loss(logits, y)
        return {f'{prefix}_loss': loss}

and we can train this using the same trainer

autoencoder = AutoEncoder()
trainer = Trainer()
trainer.fit(autoencoder)

And remember that the forward method is to define the practical use of a LightningModule. In this case, we want to use the AutoEncoder to extract image representations

some_images = torch.Tensor(32, 1, 28, 28)
representations = autoencoder(some_images)
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