fix(utils): preserve exc_info/stack_info in init_logging formatter (#4215)

* fix(utils): preserve exc_info/stack_info in init_logging formatter

Replacing Formatter.format dropped logging.exception() tracebacks,
hurting HIL-SERL actor/learner crash diagnosis. Append formatted
exceptions and stack_info like the stdlib formatter.

Fixes #3978

* refactor(utils): format logging

---------

Co-authored-by: Bartok9 <danielrpike9@gmail.com>

src/lerobot/utils/utils.py

@@ -24,7 +24,6 @@ import sys
 import time
 from collections.abc import Iterator
 from copy import copy, deepcopy
-from datetime import datetime
 from pathlib import Path
 from statistics import mean
 from typing import TYPE_CHECKING, Any
@@ -61,14 +60,16 @@ def init_logging(
         accelerator: Optional Accelerator instance (for multi-GPU detection)
     """
 
-    def custom_format(record: logging.LogRecord) -> str:
-        dt = datetime.now().strftime("%Y-%m-%d %H:%M:%S")
-        fnameline = f"{record.pathname}:{record.lineno}"
-        pid_str = f"[PID: {os.getpid()}] " if display_pid else ""
-        return f"{record.levelname} {pid_str}{dt} {fnameline[-15:]:>15} {record.getMessage()}"
+    class LeRobotFormatter(logging.Formatter):
+        def format(self, record: logging.LogRecord) -> str:
+            record.lerobot_location = f"{record.pathname}:{record.lineno}"[-15:]
+            record.lerobot_pid = f"[PID: {os.getpid()}] " if display_pid else ""
+            return super().format(record)
 
-    formatter = logging.Formatter()
-    formatter.format = custom_format
+    formatter = LeRobotFormatter(
+        "%(levelname)s %(lerobot_pid)s%(asctime)s %(lerobot_location)15s %(message)s",
+        datefmt="%Y-%m-%d %H:%M:%S",
+    )
 
     logger = logging.getLogger()
     logger.setLevel(logging.NOTSET)