Remove unimported `dist.` from tensor shapes tutorial (#2406)
Remove unimported `dist.` from tensor shapes tutorial (#2406)

At the top of the notebook, `Normal` is already imported as
```
from pyro.distributions import Bernoulli, Categorical, MultivariateNormal, Normal
```
, so it's OK to just write `Normal`, rather than `dist.Normal` (especially as `dist` hasn't been imported).

As an aside, thanks for your phenomenal tutorials, they're really well-written and educational!
uber/pyro,uber/pyro,uber/pyro