set followlinks on posix platform (#716)

Author: ShenYuhan

visualdl/io/bfile.py

@@ -124,6 +124,8 @@ def write(self, filename, file_content, binary_mode=False):
         self._write(filename, file_content, "wb" if binary_mode else "w")
 
     def walk(self, dir):
+        if 'posix' == os.name:
+            return os.walk(dir, followlinks=True)
         return os.walk(dir)