dbr quant overhead[9/x]: precalculate when to skip op_convert_after_hook (#68432)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/68432
Speeds up `op_convert_after_hook` by precalculating when this hook is a no-op
based on informationg gathered while tracing, and skipping execution when
this flag is true.
```
MobileNetV2, function level profiling, 1x3x224x224
// before
op_convert_before_hook = 3.25%
// after
op_convert_before_hook = 1.35%
```
Test Plan:
```
python test/test_quantization.py TestQuantizeDBR
```
Reviewed By: jerryzh168
Differential Revision: D32463752
Pulled By: vkuzo
fbshipit-source-id: b0c3d37909ddc8c254fe53f90954f625ae874e3b
