Update - add import for missing modules in the example codes

[kkasemos]

Why are these changes needed?

The example code does not import some modules and classes so that it cannot run as expected, need some code changes to make it works.

Related issue number

None.

Checks

• I've included any doc changes needed for https://microsoft.github.io/autogen/. See https://microsoft.github.io/autogen/docs/Contribute#documentation to build and test documentation locally.
• I've added tests (if relevant) corresponding to the changes introduced in this PR.
• I've made sure all auto checks have passed.