Pr build.yaml enhancement

[stutiibm]

Checklist

• Have you checked and followed all the points mention in the CONTRIBUTING.MD
• Have you validated script on UBI 9 container
• Did you run the script(s) on fresh container with set -e option enabled and observe success ?
• Did you have Legal approvals for patch files ?

[stutiibm]

1. Enhanced PR-build.yaml with below changes :

• Added check_chnages job to check_changes job acts as a gatekeeper / pre-filter for the whole workflow.
  Its purpose is to determine ---->
  Whether the CI build should run at all
  Which files changed in the PR
  Whether downstream jobs should be skipped based on variable values.

• Added "set_job_control_flags" stage
  which read build settings from build_info.json
  Analyze which files changed in the PR
  Decide which jobs should run
  Export those decisions as outputs for downstream jobs

2. Modified validate_build.py :
   Commented out script execution logic - validation now only checks headers, license, and line endings and actual script execution moved to execute_changed_scripts.py in the build job

3. Added execute_changed_scripts.py :
   Separates script execution from validation to avoid duplicate builds and enable proper version parameter handling.

[stutiibm]

scenario to test :

• change only build_info.json ----------------------> link
• change only build_script ----------------------> link
• change only dockerfile ----------------------> link
• change .sh file from non-relevant folder ----------> link
• change .py file ------------------------------------> link
• change multiple .sh files from relevant folders ---> link
• check pytroch 2.7.0 version issue -----------------> link