简单放置

TensorFlow 遵循这些简单的规则，也称为简单放置，用于将变量放在设备上：

If the graph was previously run, 
    then the node is left on the device where it was placed earlier
Else If the tf.device() block is used,
    then the node is placed on the specified device
Else If the GPU is present
    then the node is placed on the first available GPU
Else If the GPU is not present
    then the node is placed on the CPU