move the A100 stage to main build (#21722)

### Description
<!-- Describe your changes. -->



### Motivation and Context
We couldn't get enough A100 agent time to finish the jobs since today.
The PR makes the A100 job only runs in main branch to unblock other PRs
if it's not recovered in a short time.

tools/ci_build/github/azure-pipelines/bigmodels-ci-pipeline.yml

@@ -282,6 +282,7 @@ stages:
 - stage: Llama2_7B_ONNX
   dependsOn:
   - Build_Onnxruntime_Cuda
+  condition: and (succeeded(), or(eq(variables['Build.SourceBranch'], 'refs/heads/main'), startsWith(variables['Build.SourceBranch'], 'refs/heads/rel-')))
   jobs:
   - job: Llama2_7B_ONNX
     timeoutInMinutes: 120