Features:
- `download_file` for downloading any file from URL with progress bar.
- `SystemStatsLogger` callback to log cpu and memory usage.
- `GpuStatsLogger` callback to log gpu utilization and gpu memory utilization.
- `NekoTrainer` now auto load `SystemStatsLogger` if `gpumonitor` is installed, and `SystemStatsLogger` if `psutil` is installed.
- Rename `util.func` as `util.misc`; `util.dispatched_func` as `util.dispatched_misc`
- `WindowMerger` for aggregating sliding window results.