How to pass external form instance to StepsFormComponent? (It seems not working)

### 🧐 问题描述

Trying to pass form instance returned by useForm hook to StepsForm component.
According to [this comment](https://github.com/ant-design/pro-components/issues/7834#issuecomment-1873326662) StepsForm should support external instance, but it's don't.

(pro-components v2.7.9)

### 🚑 其他信息

Neither type suggest (prop in StepsForm interface) and instance not working.
Here i want to watch props in all steps with Form.useWatch, but I cant cause when using useFormInstance hook in nested steps - parent StepsForm dont know about steps states. Also form prop in FormStep (not Steps) is not declared in interface, but working

Start by reproducing the reported useForm, StepsForm, and nested FormStep setup against pro-components v2.7.9, then inspect the StepsForm and FormStep interfaces and their form-instance handling. Done means the external instance is accepted by the types, works at runtime, and allows Form.useWatch to observe state across steps.

Written by the indexing model from the issue text.