observers: use clamp instead of min/max in calculate_qparams (#43150)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/43150
The current logic was expensive because it created tensors on CUDA.
Switching to clamp since it can work without needing to create tensors.
Test Plan:
benchmarks
Imported from OSS
Reviewed By: jerryzh168
Differential Revision: D23170427
fbshipit-source-id: 6fe3a728e737aca9f6c2c4d518c6376738577e21
