Support passing custom height and width for validation resolutions

[a-r-r-o-w]

When doing multi-resolution/frame training, validation only occurs for 49 x 720 x 480 videos. Since we support multiple prompts (--validation_prompts) and images as validation inputs (--validation_images) by making use of the --validation_prompt_separator, I think it also makes sense to support multiple resolutions via a --validation_resolutions parameter.

@zRzRzRzRzRzRzR @sayakpaul

[sayakpaul]

Yeah not a bad idea but the logic could get messy if I am envisioning it correctly. Better could be add a small note saying something like "since this trained using multiple resolutions, be sure to test it on multiple resolutions to confirm effectiveness."