Compatibility with both tf1.11 and tf1.6

[sj6077]

Github issue: #3

Major changes:

• Change update_control_consumers because op._recompute_node_def() is removed in tf1.11
• GLOBAL_STEP is now included in bytes_list instead of node_list
• The updated dataset with correct shard_id and num_shards is added to the graph

Minor changes to note:

• export_graph_def is added for debugging
• constant tensors in NMT input pipeline must be assigned to CPU -> debugging is necessary later(I think it is the error of TensorFlow new version)

Tests for the changes:

• You can test with r1.11 in snuspl/tensorflow

Other comments:

• After adding new dataset with updated shard id and num shards, the old dataset must be removed in the future.

resolves #3