quant: add `extra_repr` to HistogramObserver (#84760)
Summary:
Adds `extra_repr` to `HistogramObserver`. This is useful when debugging
PTQ models because it allows to quickly check whether a `HistogramObserver`
has received data or not.
Test plan:
```
>>> import torch
>>> obs = torch.ao.quantization.HistogramObserver()
>>> obs(torch.randn(1, 3, 224, 224))
...
>>> print(obs)
// before - hard to tell if observer has seen data
HistogramObserver()
// after
HistogramObserver(min_val=-4.778339862823486, max_val=4.311892986297607)
>>>
```
Pull Request resolved: https://github.com/pytorch/pytorch/pull/84760
Approved by: https://github.com/andrewor14
