message: Add additional shape validation to `compute_accidental_hits`
  In `compute_accidental_hits`, the `sampled_candidates` must
  be a vector, as is shown in the kernel implementation in
  `tensorflow/core/kernels/candidate_sampler_ops.cc`.
  This fix adds shape validation of `sampled_candidates`
  in the shape function whenever possible.
diff: @@ -145,12 +145,15 @@ REGISTER_OP("ComputeAccidentalHits")
  int64 num_true;
  TF_RETURN_IF_ERROR(c->GetAttr("num_true", &num_true));
  - // Validate true_classes.
  + // Validate true_classes, must be a matrix.
  ShapeHandle true_classes;
  TF_RETURN_IF_ERROR(c->WithRank(c->input(0), 2, &true_classes));
  DimensionHandle unused;
  TF_RETURN_IF_ERROR(
  c->WithValue(c->Dim(true_classes, 1), num_true, &unused));
  + // Validate sampled_candidates, must be a vector.
  + ShapeHandle sampled_candidates;
  + TF_RETURN_IF_ERROR(c->WithRank(c->input(1), 1, &sampled_candidates));
  // All three outputs are the same shape.
  ShapeHandle v = c->Vector(InferenceContext::kUnknownDim);
  