title: tf.contrib.lookup.HashTable issue
body: <p>I try to upgrade a cost function in which I need to calculate a lookup table:</p>
  
  <pre><code>tf_look_up_keys = tf.as_string(tf.range(0, int(N+1), dtype=tf.int32 ))
  tf_look_up_values = ....an array of N+1 elements ....
  tf.contrib.lookup.HashTable(tf.contrib.lookup.KeyValueTensorInitializer(tf_look_up_keys, tf_look_up_values), -1)
  </code></pre>
  
  <p>but when I start the session it tells this:</p>
  
  <p>tensorflow.python.framework.errors_impl.FailedPreconditionError: Table not initialized.</p>
  
  <pre><code>&gt;          [[Node: hash_table_Lookup = LookupTableFind[Tin=DT_STRING, Tout=DT_FLOAT, _class=["loc:@hash_table"],
  &gt; _device="/job:localhost/replica:0/task:0/cpu:0"](hash_table, AsString_1, hash_table/Const)]]
  </code></pre>
  
  <p>what should I do? In which way I can initialize (I tried to move up tf.global_variables_initializer() but it results in a long list of complains about other variables that before this modification were ok)
  Thank you, 
  cheers</p>