[PaddlePaddle] set get_dataloader_workers to 4 (d2l-ai#1203)

* [PaddlePaddle] set get_dataloader_workers to 4

* Rerun lr scheduler.md

* Try to resolve Termination signal

* Remove extra comments

Author: 吴高升

chapter_linear-networks/image-classification-dataset.md

@@ -256,11 +256,12 @@ batch_size = 256
 
 def get_dataloader_workers():  #@save
     """使用4个进程来读取数据"""
-    return 4 if ('cpu' in paddle.device.get_device()) else 0
+    return 4
 
 train_iter = paddle.io.DataLoader(dataset=mnist_train,
                                   batch_size=batch_size,
                                   shuffle=True,
+                                  return_list=True,
                                   num_workers=get_dataloader_workers())
 ```
 
@@ -348,9 +349,11 @@ def load_data_fashion_mnist(batch_size, resize=None):
     return (paddle.io.DataLoader(dataset=mnist_train,
                                  batch_size=batch_size,
                                  shuffle=True,
+                                 return_list=True,
                                  num_workers=get_dataloader_workers()),
             paddle.io.DataLoader(dataset=mnist_test,
                                  batch_size=batch_size,
+                                 return_list=True,
                                  shuffle=True,
                                  num_workers=get_dataloader_workers()))
 ```

d2l/paddle.py

@@ -190,7 +190,7 @@ def get_dataloader_workers():
     """使用4个进程来读取数据
 
     Defined in :numref:`sec_fashion_mnist`"""
-    return 4 if ('cpu' in paddle.device.get_device()) else 0
+    return 4
 
 def load_data_fashion_mnist(batch_size, resize=None):
     """下载Fashion-MNIST数据集，然后将其加载到内存中