Debug: Resolve the DALLE Agent's self.client error. (#1798)

* Change defualt model for `lmm`

* Try to use PIL image for LMM's _oai_messages

* Update test cases and llava

* Remove redundant files

* Update the imports for lmm tests

* Test case fix

* Docstring update

* LMM notebook lint

* Typo correction for img_utils and its test

* Fix self.client bug for Dalle Agent
The self.client member variable is reserved for OpenAI's LLM-compatible
client, which requires usage summary support.
So, we rename self.client to self._dalle_client to resolve this issue