Add support for OpenMPI Rank Assignment

🚀 Feature Request

Currently, there is no way to read the variables OMPI_COMM_WORLD_SIZE, OMPI_COMM_WORLD_RANK, and OMPI_COMM_WORLD_LOCAL_RANK to assign ranks to any individual processes launched with OpenMPI

Motivation

I'd like to launch jobs with OpenMPI without using multiple-data mode, which can cause issues with command length if I have to specify all of the ranks manually.

Pitch

Add _infer_ompi_init function in fairseq/distributed/utils.py which reads the environment variables:

MASTER_ADDR
MASTER_PORT
OMPI_COMM_WORLD_SIZE
OMPI_COMM_WORLD_RANK
OMPI_COMM_WORLD_LOCAL_RANK

and updates the distributed config accordingly.

Alternatives

N/A

Additional context

N/A