frame step support, get video clip with frame jump steps

[hudengjunai]

layer.desc.stride.x = 1 layer.desc.stride.y = width * layer.desc.stride.x layer.desc.stride.c = layer.desc.stride.y * height layer.desc.stride.n = layer.desc.stride.c * channels*frame_stride
Deep Learning in videos often choose frames with a certain steps,namely jump frames. so I think it's better to support a frame stride parameters

[mitmul]

@hudengjunai Thank you for suggesting a feature. Is that already possible with the original NVVL? This repository is just a wrapper of NVVL, so if it isn't supported by the original NVVL, adding that feature to this package is a bit hard to do.