Refa: Update LLM stream response type to Generator (#9420)
### What problem does this PR solve?
Change return type of _generate_streamly from str to Generator[str,
None, None] to properly type hint streaming responses.
### Type of change
- [x] Refactoring
### What problem does this PR solve?
#9082#6365
<u> **WARNING: it's not compatible with the older version of `Agent`
module, which means that `Agent` from older versions can not work
anymore.**</u>
### Type of change
- [x] New Feature (non-breaking change which adds functionality)
Kevin Hu
Liu An