Loss functions are binary functions that expect an input vector and a target vector and return the corresponding loss. You can either class-based loss functions like `torch.nn.CrossEntropyLoss`, or you can use normal Python methods. The framework expects the configuration object `loss_fn` to point to a Python language construct that can be called as a method with the above two arguments.
In the case that you use Pytorch's builtin loss functions inside the `torc.nn` module, you can simply point to the class via the `loss_fn._target_` key:
```yaml
loss_fn:
_target_:torch.nn.CrossEntropyLoss
```
If you want to use a normal Python method as a loss function, you have to use a `hydra` utility function to make this work properly:
