F.relu operates on a Variables containing a  Tensor object and not on a  numpy ndarray.
If you want to use torch Functions (such as F.relu), convert your numpy ndarray to a torch Tensor as:
np_arr = np.array([0,1,2,3])
tensor_arr = torch.from_numpy(np_arr)
tensor_var = Variable(tensor_arr)

F.relu(tensor_var)