Tensorflow versions of the Blitz tutorial

## 📚 Documentation

**Please specify whether it's tutorial part or API reference part:** Tutorial Part
**Describe the issue**
As part of learning how to use DGL with Tensorflow, I converted all 6 tutorial examples (provided in PyTorch) to equivalent Tensorflow versions with comparable performance. Would there be interest in incorporating these versions? I feel it might increase DGL uptake among the Tensorflow community if such a tutorial existed.

Mechanics wise, I was thinking of creating a sub-folder `blitz_tensorflow` under the `tutorials` folder (i.e. a sibling of the `blitz` folder) and replacing the code portions in each of the files in there.

Please let me know if this seems like a good idea, if so, happy to create a PR with my code and contribute back.