Exception from get_nested_field() does not stop the training

Error message is printed:

```text
E0412 17:25:58.218415 140056510449472 nest.py:61] map_structure() fails for ('input.1',). Error message: 'Cannot find path 'input.1' in nested. nested has paths: ['input']'. The paths in nests are ([''],).
```

But the code keep running. Don't know why the code didn't.

