message: binary add and zeros like update
diff: @@ -179,6 +179,7 @@ class TensorMapHasKey : public OpKernel {
  template <typename Device>
  Status TensorMapBinaryAdd(OpKernelContext* c, const TensorMap& a,
  const TensorMap& b, TensorMap* out) {
  + // binary add returns a union of keys, values with keys in the intersection are added
  out->tensors() = a.tensors();
  for (const std::pair<TensorKey,Tensor>& p : b.tensors()) {
  absl::flat_hash_map<TensorKey,Tensor>::iterator it = out->tensors().find(p.first);
  @@ -196,11 +197,7 @@ Status TensorMapBinaryAdd(OpKernelContext* c, const TensorMap& a,
  template <typename Device>
  Status TensorMapZerosLike(OpKernelContext* c, const TensorMap& x, TensorMap* y) {
  - for (const std::pair<TensorKey,Tensor>& p : x.tensors()) {
  - Tensor val;
  - TF_RETURN_IF_ERROR(ZerosLikeTensor<Device>(c, p.second, &val));
  - y->tensors().emplace(p.first, val);
  - }
  + // zeros like returns an empty map
  return Status::OK();
  }
  