modified core.py to be tensorflow:2.3.0 compliant

[pangyuteng]

Thanks for sharing this implementation!
I've made slight updates in core.py for it to be compliant with Tensorflow 2.3.0.
Updates were "tested" with your provided example script within a docker container (Dockerfile also added) using below commands.

docker build -t tsne .
docker run --gpus all -it -v ${PWD}:/opt -w /opt tsne /bin/bash
python example/example_viz_parametric_tSNE.py