fix doc issue

mindspore/train/model.py

@@ -83,7 +83,7 @@ class Model:
         >>>         return out
         >>>
         >>> net = Net()
-        >>> loss = nn.SoftmaxCrossEntropyWithLogits()
+        >>> loss = nn.SoftmaxCrossEntropyWithLogits(is_grad=False, sparse=True)
         >>> optim = Momentum(params=net.trainable_params(), learning_rate=0.1, momentum=0.9)
         >>> model = Model(net, loss_fn=loss, optimizer=optim, metrics=None)
         >>> dataset = get_dataset()
@@ -400,7 +400,7 @@ class Model:
         Examples:
             >>> dataset = get_dataset()
             >>> net = Net()
-            >>> loss = nn.SoftmaxCrossEntropyWithLogits()
+            >>> loss = nn.SoftmaxCrossEntropyWithLogits(is_grad=False, sparse=True)
             >>> loss_scale_manager = FixedLossScaleManager()
             >>> optim = Momentum(params=net.trainable_params(), learning_rate=0.1, momentum=0.9)
             >>> model = Model(net, loss_fn=loss, optimizer=optim, metrics=None, loss_scale_manager=loss_scale_manager)
@@ -523,7 +523,7 @@ class Model:
         Examples:
             >>> dataset = get_dataset()
             >>> net = Net()
-            >>> loss = nn.SoftmaxCrossEntropyWithLogits()
+            >>> loss = nn.SoftmaxCrossEntropyWithLogits(is_grad=False, sparse=True)
             >>> model = Model(net, loss_fn=loss, optimizer=None, metrics={'acc'})
             >>> model.eval(dataset)
         """