example: add backward propagation to vanilla rnn example

[raistefintel]

Description

Adding backward propagation primitive to vanilla_rnn.cpp example.

Fixes # (github issue)

Checklist

General

only tested the new example with oneDNN v3.8

• [ ] Do all unit and benchdnn tests (make test and make test_benchdnn_*) pass locally for each commit?
• [x] Have you formatted the code using clang-format?

[raistefintel]

Hi @shu1chen, please check the updated version

[shu1chen]

Hello @uxlfoundation/onednn-arch, Could you please review the updated example, which now includes backward propagation for the vanilla RNN?