Shufflenet v2 pretrained

[ShawnXuan]

Added two pre-trained models. These two models are from torchvision, with original files ending in pth. The file names have been modified according to the conventions of flowvision, and they have been compressed into a zip file and uploaded to OSS.

filename	from torchvision
shufflenet_v2_x2_0	https://download.pytorch.org/models/shufflenetv2_x2_0-8be3c8ee.pth
shufflenet_v2_x1_5	https://download.pytorch.org/models/shufflenetv2_x1_5-3c479a10.pth

In flowvision/models/utils.py, these two lines will create an empty directory, which will cause the failure of decompression. Therefore, I have removed these two lines.

        if not os.path.exists(extracted_file):
            os.mkdir(extracted_file)