add comments for dygraph behavior of dot op (#23874)

python/paddle/tensor/linalg.py

@@ -440,6 +440,7 @@ def dot(x, y, name=None):
 
     """
     op_type = 'dot'
+    # skip var type check in dygraph mode to improve efficiency
     if in_dygraph_mode():
         op = getattr(core.ops, op_type)
         return op(x, y)