class_docstr: A placeholder VariantLabeler.
  
  This class provides a label_variants implementation and so allows the base
  class to be instantiated and its methods tested.
class_docstr_tok: ["A","placeholder","VariantLabeler",".","This","class","provides","a","label_variants","implementation","and","so","allows","the","base","class","to","be","instantiated","and","its","methods","tested","."]
code: def label_variants(self, variants, region):
      raise NotImplementedError
  
imports: from absl.testing import absltest
  from absl.testing import parameterized
  from third_party.nucleus.io import vcf
  from third_party.nucleus.testing import test_utils
  from third_party.nucleus.util import ranges
  from deepvariant import testdata
  from deepvariant.labeler import variant_labeler
label_desc: Use image node_id 2 for calling the PlaceholderVariantLabeler obj's underlying member method code with example usage: obj.label_variants(variants, region) without return types