fitView=true not working 图没有自动适配画布大小

**Describe the bug**
添加了options={{fitView: true}}，但是画布并没有缩放到显示所有节点。

参考链接：https://www.yuque.com/antv/g6/graph#2VtUQ

**To Reproduce**
比如这个示例：
https://codesandbox.io/s/layout-selector-le6s4?file=/src/index.js

**Expected behavior**
添加了options={{fitView: true}}后，画布缩放到显示所有节点。

**Desktop (please complete the following information):**
- OS: [e.g. iOS]
- Browser: chrome
- Version: 83.0.4103.116

Start with the reproduced example in the linked CodeSandbox, especially src/index.js, and compare its options={{fitView: true}} behavior with the referenced G6 documentation. Trace how the Graphin example handles fitView and verify the result in Chrome. Done means the canvas automatically scales to display all nodes when fitView is enabled.

Written by the indexing model from the issue text.