add model selection for nlg

[liususan091219]

Why are these changes needed?

Related issue number

Checks

• I've used pre-commit to lint the changes in this PR, or I've made sure lint with flake8 output is two 0s.
• I've included any doc changes needed for https://microsoft.github.io/FLAML/. See https://microsoft.github.io/FLAML/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.

[sonichi]

Do you have a test to cover this use case?

[liususan091219]

Do you have a test to cover this use case?

will do

[sonichi]

Do you have a test to cover this use case?

will do

Please resolve conflict and add test.