message: Assert is a Noop in xla, enable for strings
  Summary:
  Similiar to cpu/gpu
  required for birnn with a dynamic sequence length.
  Test Plan: none
  Reviewers: #tensorflow, jakeh
diff: @@ -42,6 +42,9 @@ static bool OpFilter(KernelDef* kdef) {
  if (kdef->op() == "Qr") return false;
  if (kdef->op() == "Real") return false;
  + if (kdef->op() == "Assert") {
  + AddDtypeToKernelDefConstraint("T", DT_STRING, kdef);
  + }
  if (kdef->op() == "Const") {
  AddDtypeToKernelDefConstraint("dtype", DT_STRING, kdef);
  }
  