<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def _value_function(self, x_input, y_true, y_pred):
 """ Return classification accuracy of input """

        if len(y_true.shape) == 1:
            return y_pred.argmax(1).eq(y_true).double().mean().item()
        else:
            raise NotImplementedError