enable default cascade for template.coloraxis.colorscale in plotly express

The official way of controlling the PX colorscale via templates is like this:

import plotly.express as px
import plotly.graph_objects as go

template = go.layout.Template()
template.layout.colorscale.sequential = px.colors.sequential.Blues

px.imshow([[1, 20, 30], [20, 1, 60], [30, 60, 1]], template=template)

But using the following will cause PX to use Viridis because we're providing a template without a default colorscale in the above location... This is pretty counter-intuitive and a relatively reasonable defaults cascade seems possible.

import plotly.express as px
import plotly.graph_objects as go

template = go.layout.Template()
template.layout.coloraxis.colorscale= px.colors.sequential.Blues

px.imshow([[1, 20, 30], [20, 1, 60], [30, 60, 1]], template=template)

首先重现 issue 中的两个 Plotly Express 示例，并比较生成的颜色刻度。跟踪 PX 如何应用 template.layout.colorscale.sequential 和 template.layout.coloraxis.colorscale，然后为预期的级联行为添加覆盖。完成标准是第二个示例使用所提供的 Blues 颜色刻度，而不是 Viridis。