sorry, i can not reproduce your error.
>>> loss_1 = torch.tensor([0.88])
>>> loss_2 = torch.tensor([1.88])
>>> loss_3 = torch.tensor([2.88])
>>> val_loss = [loss_1.data, loss_2.data, loss_3.data]
>>> import numpy as np
>>> l = np.mean(np.array(val_loss))
>>> l
1.8800001
>>> loss_1.data
tensor([ 0.8800])