message: Fix reference lifetime issue.
  Summary: Was taking a reference of a variable that leaves scope.
  Test Plan: All CI tests pass
  Reviewers: #tensorflow, stephenm
diff: @@ -179,10 +179,9 @@ class IpuHostEmbeddingOp : public AsyncOpKernel {
  */
  Rendezvous::DoneCallback CreateIndexLookupRecvCallback(
  OpKernelContext* ctx, Rendezvous* rendezvous,
  - const Rendezvous::ParsedKey& recv_key,
  - const Rendezvous::ParsedKey& send_key, Tensor& inp,
  - AsyncOpKernel::DoneCallback done) {
  - return [this, ctx, rendezvous, recv_key, &send_key, &inp, done](
  + Rendezvous::ParsedKey recv_key, Rendezvous::ParsedKey send_key,
  + Tensor inp, AsyncOpKernel::DoneCallback done) {
  + return [this, ctx, rendezvous, recv_key, send_key, inp, done](
  const Status& status, const Rendezvous::Args& sender_args,
  const Rendezvous::Args& reciever_args, const Tensor& tensor,
  const bool is_dead) mutable {
  @@ -240,10 +239,9 @@ class IpuHostEmbeddingOp : public AsyncOpKernel {
  Rendezvous::DoneCallback CreateIndexUpdateRecvCallback(
  OpKernelContext* ctx, Rendezvous* rendezvous,
  - const Rendezvous::ParsedKey& indices_key,
  - const Rendezvous::ParsedKey& grad_key, Tensor& inp,
  - AsyncOpKernel::DoneCallback done) {
  - return [this, ctx, rendezvous, indices_key, &grad_key, &inp, done](
  + Rendezvous::ParsedKey indices_key, Rendezvous::ParsedKey grad_key,
  + Tensor inp, AsyncOpKernel::DoneCallback done) {
  + return [this, ctx, rendezvous, indices_key, grad_key, inp, done](
  const Status& status, const Rendezvous::Args& sender_args,
  const Rendezvous::Args& reciever_args, const Tensor& tensor,
  const bool is_dead) mutable {
  @@ -265,10 +263,9 @@ class IpuHostEmbeddingOp : public AsyncOpKernel {
  Rendezvous::DoneCallback CreateGradUpdateRecvCallback(
  OpKernelContext* ctx, Rendezvous* rendezvous,
  - const Rendezvous::ParsedKey& indices_key,
  - const Rendezvous::ParsedKey& grad_key, Tensor& inp, Tensor indices,
  - AsyncOpKernel::DoneCallback done) {
  - return [this, ctx, rendezvous, indices_key, &grad_key, &inp, indices, done](
  + Rendezvous::ParsedKey indices_key, Rendezvous::ParsedKey grad_key,
  + Tensor inp, Tensor indices, AsyncOpKernel::DoneCallback done) {
  + return [this, ctx, rendezvous, indices_key, grad_key, inp, indices, done](
  const Status& status, const Rendezvous::Args& sender_args,
  const Rendezvous::Args& reciever_args, const Tensor& grads,
  const bool is_dead) mutable {
  