sources:
image captioning: https://skymind.ai/wiki/lstm
https://arxiv.org/pdf/1609.06782.pdf
https://colah.github.io/posts/2015-08-Understanding-LSTMs/
https://www.analyticsvidhya.com/blog/2018/04/solving-an-image-captioning-task-using-deep-learning/
https://www.hindawi.com/journals/mpe/2018/3125879/