Fjsp

[LTluttmann]

Description

Adding environment for the Flexible Job-Shop Scheduling Problem (FJSP) and the heterogeneous graph neural network (HGNN) described in Song et al. (https://ieeexplore.ieee.org/document/9826438) to solve it. Architecture was improved by adding residual connection and batch normalization. Average makespan on smallest instance type (10 jobs and 5 machines) is ~100 compared to ~106 reported in the paper.

Motivation and Context

closes #168

Types of changes

What types of changes does your code introduce? Remove all that do not apply:

• [ ] Bug fix (non-breaking change which fixes an issue)
• [x] New feature (non-breaking change which adds core functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to change)
• [ ] Documentation (update in the documentation)
• [ ] Example (update in the folder of examples)

Checklist

Go over all the following points, and put an x in all the boxes that apply. If you are unsure about any of these, don't hesitate to ask. We are here to help!

• [ ] My change requires a change to the documentation.
• [x] I have updated the tests accordingly (required for a bug fix or a new feature).
• [ ] I have updated the documentation accordingly.

[LTluttmann]

thanks a ton for the valuable comments @fedebotu & @cbhua! I hope I addressed all issues. Merging now