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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def _export_signatures(meta_graph):
 """Exports signatures from current graph into a MetaGraphDef."""

  named_signatures = tf_v1.get_collection(_SIGNATURE_COLLECTION)
  if not named_signatures:
    raise ValueError("No signatures present. Please call hub.add_signature(...)"
                     "at least once in the module_fn.")
  for key, signature in named_signatures:
    meta_graph.signature_def[key].CopyFrom(signature)