Typo in try except block (tqdm)

```python
try:
    from tqdm.notebook import tqdm
except:
    tdqm = lambda x: x 
``` 

It should be `tqdm` everywhere. (Cell after "Cool! We've trained our model on 4 trajectories. Let's see if we can do a bit more. In particular, let's train our model with  β=32  to get it to be a bit greedy." in the getting started tutorial.