From your posted code it looks like m0 is a nn.Conv2d layer.
This should usually work:
conv = nn.Conv2d(3, 6 ,3, 1, 1)
idx0 = np.array([0, 1, 2])
conv.weight.data[:, idx0, :, :].clone()

Could you wrap it into a torch.LongTensor and see, if it helps? (torch.from_numpy(idx0))
What PyTorch version are you using?