UCX environment parameters

• Setting the devices to use from the command line.

In order to specify the devices to use for the run, please use the following environment parameters:
UCX_NET_DEVICES
UCX_SHM_DEVICES
UCX_ACC_DEVICES

UCX_NET_DEVICES for specifying the network devices. For example: mlx5_1:1: / mlx5_1:1 /
UCX_SHM_DEVICES for specifying the shared memory devices. For example: sysv / knem
UCX_ACC_DEVICES for specifying the acceleration devices. For example: gpu0

The following command line will use the rc_x and mm UCTs and their corresponding devices will be mlx5_0:1 and sysv.
mpirun -mca pml ucx -x UCX_TLS=rc_x,mm -x UCX_NET_DEVICES=mlx5_0:1 -x UCX_SHM_DEVICES=sysv ...

This way, for instance, making the choice for the HCA to use doesn't affect the devices used for the shared memory UCTs.