message: Make TpuContext thread-local.
  tf.contrib.AsyncCheckpointSaverHook creates a thread to asynchronously run the export, which currently breaks when exporting a TPU graph.
diff: @@ -20,9 +20,10 @@ from __future__ import division
  from __future__ import print_function
  import contextlib
  +import threading
  -class TpuContext(object):
  +class TpuContext(threading.local):
  """A context object holding state about the TPU computation being built."""
  def __init__(self):
  