[Fix] Wrap log_path with Path (deepmodeling#4117)

`FLAGS.log_path` need to be wrapped into `Path` type, or will raise
error when call `.parent`.

<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->
## Summary by CodeRabbit


- **Improvements**
- Enhanced handling of log file paths for better compatibility and
functionality in file operations.

<!-- end of auto-generated comment: release notes by coderabbit.ai -->

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Signed-off-by: HydrogenSulfate <490868991@qq.com>

deepmd/pt/entrypoints/main.py

@@ -555,7 +555,11 @@ def main(args: Optional[Union[List[str], argparse.Namespace]] = None):
     else:
         FLAGS = args
 
-    set_log_handles(FLAGS.log_level, FLAGS.log_path, mpi_log=None)
+    set_log_handles(
+        FLAGS.log_level,
+        Path(FLAGS.log_path) if FLAGS.log_path else None,
+        mpi_log=None,
+    )
     log.debug("Log handles were successfully set")
     log.info("DeePMD version: %s", __version__)