quant: switch observers to use min_max (#42957)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/42957
Switches observers to use the new min_max function to calculate
min and max at the same time. We see around 45-50% speedup on
representative input shapes on the microbenchmarks for all observers except `HistogramObserver`.
Test Plan:
CI for correctness
performance:
```
cd benchmarks/operator_benchmark
// repeat (before diff, after diff) x (cpu, cuda)
python -m pt.qobserver_test --tag_filter all --device cpu
/*
* before, cpu: https://our.intern.facebook.com/intern/paste/P138633280/
* before, cuda: https://our.intern.facebook.com/intern/paste/P138639473/
* after, cpu: https://our.intern.facebook.com/intern/paste/P138635458/
* after, cuda: https://our.intern.facebook.com/intern/paste/P138636344/
*/
```
Imported from OSS
Reviewed By: supriyar
Differential Revision: D23093995
fbshipit-source-id: 9f416d144109b5b80baf089eb4bcfabe8fe358d5
