[Core/RLlib] Move log_once from rllib to ray.util. (#7273)

* Move log_once from rllib to tune.

* Move log_once from rllib to tune.

* LINT.

* Move to ray.util.debug.

python/ray/util/debug.py

@@ -0,0 +1,46 @@
+import time
+
+_logged = set()
+_disabled = False
+_periodic_log = False
+_last_logged = 0.0
+
+
+def log_once(key):
+    """Returns True if this is the "first" call for a given key.
+
+    Various logging settings can adjust the definition of "first".
+
+    Example:
+        >>> if log_once("some_key"):
+        ...     logger.info("Some verbose logging statement")
+    """
+
+    global _last_logged
+
+    if _disabled:
+        return False
+    elif key not in _logged:
+        _logged.add(key)
+        _last_logged = time.time()
+        return True
+    elif _periodic_log and time.time() - _last_logged > 60.0:
+        _logged.clear()
+        _last_logged = time.time()
+        return False
+    else:
+        return False
+
+
+def disable_log_once_globally():
+    """Make log_once() return False in this process."""
+
+    global _disabled
+    _disabled = True
+
+
+def enable_periodic_logging():
+    """Make log_once() periodically return True in this process."""
+
+    global _periodic_log
+    _periodic_log = True