nb2 06_cnn_cuda_hooks_init.ipynb plotting errors

plotting the means and std dev as given in the notebooks, for example:
```python
for o in act_means: plt.plot(o)
plt.legend(range(5));
```
gives an error as o is a list of lists

to correct this, we can use 

```python
for o in act_means: plt.plot(torch.tensor(o))
plt.legend(range(5));
```

