First, create a conda environment with
conda env create -f fairseq.yml
I'm using torch_scatter, which doesn't seem to install directly with conda.
Then, install my fork of fairseq from pstjohn/fairseq with pip install --editable ./ in the resulting conda environment.
On eagle, my environment is kept in /projects/deepgreen/pstjohn/envs/fairseq. Use --prefix=... during conda env create to specify a custom directory.