message: Fix opaque/backend config regression
diff: @@ -890,7 +890,7 @@ ENTRY c1 {
  p0 = (s32[20], s32[20], s32[20], s32[20]) parameter(0)
  p0_0 = s32[20] get-tuple-element(p0), index=0
  - c = s32[20] custom-call(p0), custom_call_target="Popnn::LstmLayerFwd", opaque="{\"num_channels\":4, \"is_training\":false, \"partials_dtype\":\"DT_FLOAT\"}\n"
  + c = s32[20] custom-call(p0), custom_call_target="Popnn::LstmLayerFwd", backend_config="{\"num_channels\":4, \"is_training\":false, \"partials_dtype\":\"DT_FLOAT\"}\n"
  ROOT a = s32[20] add(p0_0, c)
  }
  @@ -904,7 +904,7 @@ ENTRY c1 {
  p0 = (s32[20], s32[20], s32[20], s32[20]) parameter(0)
  p0_0 = s32[20] get-tuple-element(p0), index=0
  - c = s32[20] custom-call(p0), custom_call_target="Popnn::LstmLayerFwd", opaque="{\"num_channels\":4, \"is_training\":false, \"partials_dtype\":\"DT_FLOAT\"}\n"
  + c = s32[20] custom-call(p0), custom_call_target="Popnn::LstmLayerFwd", backend_config="{\"num_channels\":4, \"is_training\":false, \"partials_dtype\":\"DT_FLOAT\"}\n"
  ROOT a = s32[20] add(p0_0, c)
  }
  