message: Fix device syntax in `_TrackableResourceSummaryWriter`
  It turns out `cpu:0` is not technically allowed and just happens to be accepted by the Python parsing code, but causes issues loading the SavedModel from C++. We can avoid that by using the fuller form `/CPU:0`.
diff: @@ -363,7 +363,7 @@ class _TrackableResourceSummaryWriter(_ResourceSummaryWriter,
  def __init__(self, create_fn, init_op_fn):
  # Resolve multiple inheritance via explicit calls to __init__() on parents.
  - tracking.TrackableResource.__init__(self, device="cpu:0")
  + tracking.TrackableResource.__init__(self, device="/CPU:0")
  self._create_fn = create_fn
  self._init_op_fn = init_op_fn
  # Pass .resource_handle into _ResourceSummaryWriter parent class rather than
  