fix: remove classmethod from `Qwen2_5OmniConfig.get_text_config` (#37690)
- Since the `get_text_config` references an instance variable within
the class (`self.thinker_config`), the `get_text_config` method
should not be a classmethod.
- Before this fix, users were getting the following error:
'''
AttributeError: type object 'Qwen2_5OmniConfig' has no attribute 'thinker_config'
'''