Initialize lines = [] in load_log_files to avoid UnboundLocalError on FileNotFoundError retry failures

[Chessing234]

Bug:
In fastchat/serve/monitor/basic_stats.py, when all 5 retries to read a log file fail due to a FileNotFoundError, an UnboundLocalError: local variable 'lines' referenced before assignment is raised at for l in lines:.

Root Cause:
lines is only assigned a value when open(filename).readlines() executes successfully inside the try block. If a FileNotFoundError occurs for all 5 attempts, the loop terminates without lines being initialized, but the code still proceeds to access lines.

Why Fix is Correct:
By initializing lines = [] before starting the retry loop, we ensure that lines is always defined. If all retries fail, it remains an empty list, and the function gracefully skips the parsing loop and returns an empty list rather than crashing.