Ignore mypy cache, add separate generate data option, update readme.

[ninjaguardian]

Changes .gitignore to ignore .mypy_cache generate_data.py was not working for me, so I made a separate option that uses the tensorflow MNIST dataset. I updated README.md to say generate_data.py instead of generate_dataset.py. I also mentioned generate_dataset_tensorflow.py, though it may be more helpful to do it as a command at the like python3 generate_data.py --mnist tensorflow but I don't know how to do that.