AttributeError: 'FocalLoss' object has no attribute 'backward'

This error occurs when you try to call the `backward()` method on an instance of the `FocalLoss` class, which does not have this method defined.

The `backward()` method is usually defined in PyTorch's `nn.Module` class, which is the base class for all neural network modules in PyTorch. If you are implementing a custom loss function by subclassing `nn.Module`, you need to define the `forward()` method to compute the loss and the `backward()` method to compute the gradients of the loss with respect to the inputs.

To fix this error, you can either define the `backward()` method in your `FocalLoss` class, or you can use one of the built-in loss functions that already have the `backward()` method implemented.