Center Title by default for px.imshow

Issue:

Many chart-making functions such as px.line, px.scatter, px.bar center the title by default (desired), but the px.imshow function displays the title on the left margin instead (not desired).

import plotly.express as px

fig = px.imshow(mat, title="My Title")

Current Workaround:

fig.update_layout(title={'text': "My Title", 'x':0.485, 'xanchor': 'center'}) # manually center the title

Proposed Solution:

The px.imshow function should center the title by default.

从 issue 中链接的 px.imshow 入口点开始，将其标题处理方式与 px.line、px.scatter 和 px.bar 进行比较。检查现有的 Plotly Express 图像 figure 测试，然后更新覆盖范围，使标题默认居中，并验证是否生成了预期的 layout。