Fix Critical Error While Loading Data

[AlonzoLeeeooo]

Hi @huxiuhan ,
I've found some critical issues of this code when debugging it, where I propose this PR to fix it. Details are below:

Previous version of the __getitem__ in animatediff/data/dataset.py, line 67-73:

 while True:
            try:
                pixel_values, name = self.get_batch(idx)
                break

            except Exception as e:
                idx = random.randint(0, self.length-1)
pixel_values = self.pixel_transforms(pixel_values)

Fixed version:

pixel_values, name = self.get_batch(idx)
pixel_values = self.pixel_transforms(pixel_values)

The previous version of dataset.py might cause dead loop if the WebVid10M dataset is not downloaded completely. Specifically, if self.get_batch(idx) does not function well, the data loading process will keep excecuting idx = random.randint(0, self.length-1), untill the idx data is available.

This might cause much inconvenicence for two reasons: (1) If one is attempting to debug this code with a tiny subset of WebVid, he might not need the entire dataset to do so; (2) The dead loop prevents us to see where the code issue truly lies.

I would really appreciate it if you can accept this PR, since the issue has been addressed in a series of issues, particularly in #201, #352, #331. Thank you so much in advance for your precious time and attention from your busy schedule.

Best regards,
Chang