Make A100 jobs in PR checks again (#22261)
### Description
if the variable is 1, the job running on A100 in PR checks.
Fixes
[AB#50333](https://aiinfra.visualstudio.com/6a833879-cd9b-44a4-a9de-adc2d818f13c/_workitems/edit/50333)
### Motivation and Context
We wish more big models which need to run on A100 can be tested in PR
checks, but Azure may decommission A100 agents without notifications
sometimes, which will block merging PRs.
This PR is an improvement of current workaround, making those jobs only
run main branch.
Once we find the A100 are all decommisioned by Azure, we could change
the UseA100 variable to 0 to disable the A100 jobs in PR checks
