dbr quant: make fqn during prepare op hook required (#69726)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/69726
This is a cleanup, this variable was previously optional
but it always exists, because the only way an op hook
can run if there is a parent module with an `AutoQuantizationState`
object.
Test Plan:
```
python test/test_quantization.py TestQuantizeDBR
```
Reviewed By: albanD
Differential Revision: D33003472
Pulled By: vkuzo
fbshipit-source-id: de5769194808d42b025b848667815b4e3d73b6c6
