Use debug logging for early stopping decisions that get filtered downstream (facebook#4652)

Summary:
Pull Request resolved: facebook#4652

The early stopping strategy was logging at info level for routine decisions, cluttering logs during optimization runs. Additionally, when `should_early_stop` returns False, the trial index and reason messages are filtered out downstream, so these messages were never being surfaced anyway. Changed logging to debug level to reduce noise while still preserving the information for debugging when needed.

Reviewed By: saitcakmak

Differential Revision: D88914755

fbshipit-source-id: 47b0dd88e6408eeecfb1770f8541ce4047bf6195

Author: ltiao

ax/early_stopping/strategies/percentile.py

@@ -295,7 +295,7 @@ def _should_stop_trial_early(
         if self.min_curves is not None:
             is_insufficient = window_num_active_trials < self.min_curves
             if is_insufficient.any():
-                return False, (
+                reason = (
                     f"Insufficiently many trials with data at progressions in window "
                     f"[{window_start:.2f}, {window_end:.2f}]\n"
                     f"- Progressions: {window_values[is_insufficient].index.tolist()}\n"
@@ -305,6 +305,8 @@ def _should_stop_trial_early(
                     f"{window_active_trial_indices[is_insufficient].tolist()}\n"
                     f"- Minimum required: {self.min_curves}"
                 )
+                logger.debug(reason)
+                return False, reason
 
         # Check if trial is in top n_best_trials_to_complete
         # and should be protected from early stopping
@@ -329,7 +331,7 @@ def _should_stop_trial_early(
                     f"- Progressions: {window_values.index.tolist()}\n"
                     f"- Best trials: {window_best_trials.tolist()}"
                 )
-                logger.info(reason)
+                logger.debug(reason)
                 return False, reason
 
         # Calculate the percentile threshold for each progression.
@@ -363,8 +365,6 @@ def _should_stop_trial_early(
             f"- Number of trials: {window_num_active_trials.tolist()}\n"
             f"- Trial indices: {window_active_trial_indices.tolist()}"
         )
-
-        if should_early_stop:
-            logger.info(f"Early stopping trial {trial_index}: {reason}.")
+        logger.debug(f"Early stopping trial {trial_index}: {reason}.")
 
         return should_early_stop, reason