GitLab

* Doc: HetGConv note for `edge_type_subgraph`

For the rational behind this note see the discussion in #3003.

* WIP: Requirements for building the docs.

Had to install all these packages to get further with building the docs.

Didn't successfully build them yet, tho.

* Test

* Update

* Update hetero.py

* Update hetero.py
Co-authored-by: Ubuntu <ubuntu@ip-172-31-13-32.us-west-2.compute.internal>
Co-authored-by: Mufei Li <mufeili1996@gmail.com>