Requirements.txt and environment.yml for nf_tutorial_torch.ipynb

[ivanmilevtues]

I forked the repo in order to play around with the Normalizing flows python notebook and noticed that as of the moment there is no requirements file as the notebook is adjusted for Google Colab.

What is done in this pull request:

1. environment.yml - creates a fresh Conda environment with everything required to run the Normalizing flow notebook and nothing more
2. requirements.txt - all the requirements if someone wants to install them with pip

Files were generated with:

• conda env export --no-builds | grep -v "prefix" > environment.yml
• conda list -e > requirements.txt