quant tests: fix log spew for HistogramObserver (#70107)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/70107
Histogram observer used floor division on tensors, which is a deprecated
behavior. There was a warning printed:
```
/Users/vasiliy/pytorch/torch/ao/quantization/observer.py:905: UserWarning: __floordiv__ is deprecated, and i
ts behavior will change in a future version of pytorch. It currently rounds toward 0 (like the 'trunc' funct
ion NOT 'floor'). This results in incorrect rounding for negative values. To keep the current behavior, use
torch.div(a, b, rounding_mode='trunc'), or for actual floor division, use torch.div(a, b, rounding_mode='flo
or').
```
This PR fixes the warning.
Test Plan:
```
python test/test_quantization.py TestHistogramObserver
```
Reviewed By: ejguan
Differential Revision: D33187926
Pulled By: vkuzo
fbshipit-source-id: 9c37de4c6d6193bee9047b6a28ff37ee1b019753
