[Fix] Add method to prepare extra step kwargs for scheduler in xFuserCogVideoXPipeline

[LazyBusyYang]

What's the problem

self.scheduler is an instance of xFuserDDIMSchedulerWrapper, whose step is defined like def step(self, *args, **kwargs). The original accepts_eta and accepts_generator are always false because set(inspect.signature(self.scheduler.step).parameters.keys()) returns a set of args and kwargs.

This update introduces the prepare_extra_step_kwargs method, which prepares additional keyword arguments for the scheduler step based on its signature. It checks for the presence of 'eta' and 'generator' parameters to ensure compatibility with different schedulers. This enhancement improves the flexibility and usability of the pipeline.