Fix a bug in get_vdl_log_file when logdir does not exist (#1002)

visualdl/server/serve.py

@@ -64,11 +64,11 @@ def get_vdl_log_file(logdirs):
                             "exp2": "vdlrecords.1587375685.log"}
     """
     walks = {}
+    walks_temp = {}
     for logdir in logdirs:
         for root, dirs, files in bfile.walk(logdir):
             walks.update({root: files})
 
-            walks_temp = {}
     for run, tags in walks.items():
         tags_temp = [tag for tag in tags if
                      is_VDLRecord_file(path=bfile.join(run, tag), check=False)]