video input 'image_aspect_ratio=pad' not work
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**Describe the bug**

Hi, 
Please check the this [line](https://github.com/NVIDIA/NeMo/blob/main/nemo/collections/multimodal/data/neva/neva_dataset.py#L980) and this [line](https://github.com/NVIDIA/NeMo/blob/main/nemo/collections/multimodal/parts/utils.py#L527)

The input to the `expand2square` is expected to be an image object but now in the video preprocessing part, it's a list of images so it will raise issue.


