Fix: bugs when save sample in `opensora.datasets.utils`

[HaiyiMei]

In the original version, the input of opensora.datasets.save_sample would be affected by the function.

For example:

import torch
from opensora.datasets import save_sample

video = torch.zeros(3, 32, 1024, 1024)
save_sample(video, save_path="video")

print(video.max())

The result should be 0, but the result from the original version was 0.5.