`autodiff`: improve the Graphviz visualisations.

[avhz]

There are problems with the Graphviz plotting of the computation graphs in autodiff:

• [ ] The nodes don't indicate the operation that took place (e.g. +, sin).
• [ ] There are double links from one node to others (e.g. input x1 has two edges to Op_3).
  • The edge should not be drawn if the weight is 0.

[nicola-corbellini]

Hello! Along my path to start learning Rust I've come across your repository and I'd like to tackle this issue if it's not out of the scope of my current skills.

May I ask for a hint? Where could I get the operation performed on the input? I haven't found it available either in the Variable nor in the Graph. Thank you

[avhz]

Hi thanks for the interest! Unfortunately there's no current way to retrieve the operation performed, and this would need to be added.

[JainVidit12]

Created Pull request #131 to address this.

[0xJepsen]

Might i recommend the plotters library as well? We have used it quite a bit and found it to be really good.