message: Use Arena for SequencExample allocation in example_parsing_ops.cc
  Provides better memory locality which speeds up most ops, and makes destruction trivial.
diff: @@ -591,7 +591,22 @@ class ParseSingleSequenceExampleOp : public OpKernel {
  OP_REQUIRES_OK(ctx, ctx->output_list("feature_list_dense_values",
  &feature_list_dense_values));
  +#ifdef TENSORFLOW_LITE_PROTOS
  SequenceExample ex;
  +#else
  + // Allocate the SequenceExample on an arena. Provides better memory locality
  + // and greatly speeds up destruction.
  + protobuf::ArenaOptions options;
  + // We have some hint of what the final proto size will be based on the size
  + // of the serialized bytes- use this to set a custom allocation strategy.
  + // Note that the default allocation strategy is quite conservative (min
  + // block size of 256 bytes, and a max of 8 kilobytes).
  + const size_t block_size = serialized_t().size() * 1.1;
  + options.start_block_size = std::max(options.start_block_size, block_size);
  + options.max_block_size = std::max(options.max_block_size, block_size);
  + protobuf::Arena arena(options);
  + auto& ex = *protobuf::Arena::CreateMessage<SequenceExample>(&arena);
  +#endif
  OP_REQUIRES(
  ctx, ParseProtoUnlimited(&ex, serialized_t()),
  errors::InvalidArgument("Could not parse example input, value: '",
  