[BUG] Broken link to http://deeplearning.net/data/mnist

### Add Link

https://pytorch.org/tutorials/beginner/nn_tutorial.html 

### Describe the bug

Linkcheck breaks for nn tutorials with:
```
 [TIMEOUT] http://deeplearning.net/data/mnist/ | Timeout
```
That link does not exist anymore - need to find a proper link to mnist dataset. Or can just delete the link.

### Describe your environment

n/a

cc @brycebortree @sekyondaMeta @AlannaBurke