Plot metrics collected during training.

def plot_metrics(self, skip_start:int=0, skip_end:int=0, return_fig:bool=None)->Optional[plt.Figure]:
        "Plot metrics collected during training."
        assert len(self.metrics) != 0, "There are no metrics to plot."
        fig, axes = plt.subplots(len(self.metrics[0]),1,figsize=(6, 4*len(self.metrics[0])))
        val_iter = self._split_list_val(np.cumsum(self.nb_batches), skip_start, skip_end)
        axes = axes.flatten() if len(self.metrics[0]) != 1 else [axes]
        for i, ax in enumerate(axes):
            values = [met[i] for met in self.metrics]
            values = self._split_list_val(values, skip_start, skip_end)
            ax.plot(val_iter, values)
            ax.set_ylabel(str(self.metrics_names[i]))
            ax.set_xlabel('Batches processed')             
        if ifnone(return_fig, defaults.return_fig): return fig
        if not IN_NOTEBOOK: plot_sixel(fig)