title: In TensorFlow,what's the meaning of ":0" in a Variable's name?
body: <pre><code>import tensorflow as tf
  with tf.device('/gpu:0'):
      foo = tf.Variable(1, name='foo')
      assert foo.name == "foo:0"
  with tf.device('/gpu:1'):
      bar = tf.Variable(1, name='bar')
      assert bar.name == "bar:0"
  </code></pre>
  
  <p>The above code returns true.I use <code>with tf.device</code> here to illustrate that the ":0" doesn't mean the variable lie on the specific device.So what's the meaning of the ":0" in the variable's name(foo and bar in this example)?</p>