Hypergraph: Add equations in docstrings of forward methods

[gurug-dev]

Changes addressing #168.

Notes:

• template_layer doesn't have corresponding eqns in awesome-tnn docs, I'm assuming as the name suggests, the code contains a basic template example. Skipped that.
• hnhn_layer_bis seems to be about HNHN, but a secondary class, HNHN already had updated forward docstrings, unsure what is to be done here.

[ninamiolane]

Excellent, thanks!

You're right: no equations for template_layer.

HNHN bis is a separate problem, that someone else will deal with in another PR. Good to leave it like that.

[ninamiolane]

@gurug-dev there is a lint issue. Could you run black on the repo and re-commit?
Thanks. black .

[gurug-dev]

@ninamiolane done, lint error was due to some other unrelated code, fixed now. Thank you!

[ninamiolane]

Ah, right, it was from the SAN model. Thanks!