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)

Empieza reproduciendo ambos ejemplos de Plotly Express del issue y compara las escalas de colores resultantes. Traza cómo PX aplica template.layout.colorscale.sequential frente a template.layout.coloraxis.colorscale y, después, añade cobertura para el comportamiento esperado de cascada. La tarea estará terminada cuando el segundo ejemplo use la escala de colores Blues proporcionada en lugar de Viridis.