message: Expand DistributionStrategy.group to address single variable case properly as well, in addition to a single Tensor case.
diff: @@ -946,7 +946,7 @@ class DistributionStrategy(object):
  return control_flow_ops.group(value, name=name)
  # Special handling for the common case of one op.
  v, = value
  - if isinstance(v, ops.Tensor):
  + if hasattr(v, "op"):
  v = v.op
  return v
  