code: def test_gvcf_basic_skips_iupac_ref_base(self, ref):
      caller = PlaceholderVariantCaller(0.01, 100)
      allele_counter = self.fake_allele_counter(100, [(0, 0, ref)])
      self.assertEmpty(
          list(caller.make_gvcfs(allele_counter.summary_counts()))
      )
  
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 6 for calling the VariantCallerTests obj's underlying member method code with example usage: obj.test_gvcf_basic_skips_iupac_ref_base(ref) without return types