fix tracing using a dictionary as input (#16616)
Summary:
Previously this would fail with the error message:
```
ValueError: Auto nesting doesn't know how to process an input object of type dict. Accepted types: Tensors, or lists/tuples of them
```
Turns out we're not using the line that causes this error (or a side effect of that line), so removing it fixes the issue. Also cleaned up some related dead code (cc apaszke to make sure the code isn't useful in some way)
Pull Request resolved: https://github.com/pytorch/pytorch/pull/16616
Differential Revision: D13908352
Pulled By: suo
fbshipit-source-id: 27094f1f4ea0af215b901f7ed3520e94fbc587b3
