fix global avg pool bug

[Jhonve]

refer to the latest torch documentation, here is an example:

real_features = torch.rand((36, 512))
real_features_reduced = torch.mean(real_features, dim=0)[0]
print(real_features_reduced.shape)
# torch.Size([])
real_features_reduced = torch.mean(real_features, dim=0, keepdim=False)
print(real_features_reduced.shape)
# torch.Size([512])

[Jhonve]

same with #708 #709 #714