Fix SVD image input

[TianxingWu]

Found a bug of SVD after the SV3D update:

If the input is a RGB image for SVD, The variable 'input_image' will not be assigned and raises:

UnboundLocalError: local variable 'input_image' referenced before assignment

Using copy() rather than direct assignment, or the following error will raise since the file is closed by the 'with' context manager:

TypeError: int() argument must be a string, a bytes-like object or a real number, not 'PngImageFile'

[TianxingWu]

Another suggestion is to use torchivision.io.write_video() rather than imageio.mimwrite() to save the video. The current output cannot take fps into account and is not compatible with many video players. For example:

from torchvision.io import write_video
...
write_video(video_path, vid, fps=fps_id + 1)