How do forward propagation and backward propagation work in PyTorch?
In PyTorch, forward propagation is implemented by defining a model’s network structure and computation flow. Firstly, a class that inherits from nn.Module needs to be defined, where the network structure is defined in the __init__ method and the computation flow is defined in the forward method. When the forward method of the model is called, the forward propagation process is automatically executed, meaning input data is passed through the network structure to produce output results.
Backpropagation is achieved by defining a loss function. In PyTorch, various loss functions such as torch.nn.CrossEntropyLoss and torch.nn.MSELoss are available in the torch.nn module. During training, the forward pass is first conducted to obtain the model’s output, then the value of the loss function is calculated, and finally the backward method is called to perform backpropagation, automatically computing gradients and updating model parameters.
In general, the forward propagation in PyTorch is achieved by defining the model’s network structure and computational flow, while the backward propagation is done by defining the loss function and calling the backward method. PyTorch provides a flexible and powerful framework that simplifies the implementation and training process of deep learning models.