Simple placement

TensorFlow follows the following rules for placing the variables on devices:

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