fix bug fot test_dygraph_mnist_fp16.py, test=develop (#22222) (#22536)

Author: JepsonWong

python/paddle/fluid/tests/unittests/test_dygraph_mnist_fp16.py

@@ -121,7 +121,7 @@ def test_mnist_fp16(self):
         if not fluid.is_compiled_with_cuda():
             return
         x = np.random.randn(1, 3, 224, 224).astype("float16")
-        y = np.random.randn(1, 1).astype("int64")
+        y = np.random.randint(10, size=[1, 1], dtype="int64")
         with fluid.dygraph.guard(fluid.CUDAPlace(0)):
             model = MNIST(dtype="float16")
             x = fluid.dygraph.to_variable(x)