Create a new accuracy KPI for target determination

[huydhn]

Per the discussion with @clee2000, we want to create a new accuracy KPI for target determination to drive its progress going forward. Given the information we have, we could compute this value as follows:

1. A TD miss happens when the failed tests weren't run on PR, but showed up in trunk
2. We can then compute the percentage of commits that satisfy (1)
3. The accuracy of TD will be 1 - (2)

The task would be to compute this KPI and display it on https://hud.pytorch.org/kpis

cc @ZainRizvi @clee2000 @seemethere