!10346 fix quant _Observer print bug

From: @yuchaojie
Reviewed-by: @linqingke,@liangchenghui
Signed-off-by: @linqingke

mindspore/nn/layer/quant.py

@@ -157,7 +157,7 @@ class _Observer(Cell):
         self.quant_dtype = quant_dtype
 
     def extend_repr(self):
-        s = f"dtype={self.dtype}"
+        s = f"quant_dtype={self.quant_dtype}"
         return s
 
     def construct(self):