make the `._has_subplots` method from `go.Figure` accessible

In some cases, when writings callbacks that only update a subplot of a Figure, it is useful to know if the figure will accept an append_trace to a specific location, or if add_trace should be used instead.

I use this workaround (using a try/except isn't great):

    if fig._has_subplots():
        fig.append_trace(trace, 1, 1)
    else:
        _ = fig.add_trace(trace)

If I only use fig.append_trace(trace, 1, 1), I get Exception: In order to reference traces by row and column, you must first use plotly.tools.make_subplots to create the figure with a subplot grid.

If I only use fig.add_trace(trace), the layout gets messed up.

Could it be possible to make this method public?

检查 go.Figure 的 _has_subplots 实现，以及围绕 append_trace、add_trace 和 make_subplots 的公开 Figure API。当调用方可以使用公开检查来确定行和列 trace 引用是否有效，而无需依赖 try/except 变通方案，同时保留现有的 subplot 行为时，这项工作就完成了。