Refactor active process management in app.py

Consolidate global variable declarations for active process management within the run_training and _stop_active_training functions. This change improves code clarity and ensures consistent handling of active training processes, enhancing overall functionality.

app.py

@@ -662,6 +662,7 @@ def run_training(
     max_epochs: int,
     save_every: int,
 ) -> Iterable[tuple]:
+    global _ACTIVE_PROC, _ACTIVE_PGID
     # Basic validation
     log_buf = ""
     ckpts: List[str] = []
@@ -851,7 +852,6 @@ def _stop_active_training() -> None:
     )
     # Register active process for hard stop
     with _ACTIVE_LOCK:
-        global _ACTIVE_PROC, _ACTIVE_PGID
         _ACTIVE_PROC = proc
         try:
             _ACTIVE_PGID = os.getpgid(proc.pid)