Show the `xs` (inputs) and `ys` (targets). `max_len` is the maximum number of tokens displayed.

def show_xys(self, xs, ys, max_len:int=70)->None:
        "Show the `xs` (inputs) and `ys` (targets). `max_len` is the maximum number of tokens displayed."
        from IPython.display import display, HTML
        names = ['idx','text'] if self._is_lm else ['text','target']
        items = []
        for i, (x,y) in enumerate(zip(xs,ys)):
            txt_x = ' '.join(x.text.split(' ')[:max_len]) if max_len is not None else x.text
            items.append([i, txt_x] if self._is_lm else [txt_x, y])
        items = np.array(items)
        df = pd.DataFrame({n:items[:,i] for i,n in enumerate(names)}, columns=names)
        with pd.option_context('display.max_colwidth', -1):
            display(HTML(df.to_html(index=False)))