https://www.tensorflow.org/versions/master/tutorials/mnist/tf/index.html#tensorflow-mechanics-101
において出てきた型変換.
Loss
First, the values from the labels_placeholder are converted to 64-bit integers.
...
labels = tf.to_int64(labels)
Tensor Transformations
というらしい。
https://www.tensorflow.org/versions/r0.11/api_docs/python/array_ops.html