You can replace backpropagation with the forward-forward algorithm by leveraging iterative forward passes to adjust model weights based on the maximization of likelihood instead of gradients, enabling training without backpropagation.
Here is the code snippet below:

In the above code, we are using the following key points:
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The ForwardForwardModel is a simple feedforward neural network.
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The custom forward_forward_training loop replaces gradient descent with an iterative process focused on maximizing the likelihood of outputs.
Hence, the Forward-Forward algorithm offers an alternative to backpropagation, relying on forward passes to optimize model parameters by directly maximizing a chosen objective.