class_docstr: A placeholder VariantCaller.
  
  This class provides a get_candidates implementation and so allows
  the base class to be instantiated and its methods tested.
class_docstr_tok: ["A","placeholder","VariantCaller",".","This","class","provides","a","get_candidates","implementation","and","so","allows","the","base","class","to","be","instantiated","and","its","methods","tested","."]
code: def get_candidates(self, allele_counters, sample_name):
      return None
  
imports: from unittest import mock
  from absl.testing import absltest
  from absl.testing import parameterized
  import numpy
  import numpy.testing
  from third_party.nucleus.util import variant_utils
  from third_party.nucleus.util import variantcall_utils
  from deepvariant import testdata
  from deepvariant import variant_caller
  from deepvariant.protos import deepvariant_pb2
label_desc: Use image node_id 2 for calling the PlaceholderVariantCaller obj's underlying member method code with example usage: obj.get_candidates(allele_counters, sample_name) and returns: None