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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:

def on_backward_begin(self, smooth_loss:Tensor, **kwargs:Any)->None:
        "Record the loss before any other callback has a chance to modify it."
        self.losses.append(smooth_loss)
        if self.pbar is not None and hasattr(self.pbar,'child'):
            self.pbar.child.comment = f'{smooth_loss:.4f}'