Rename "gpu_ids" parameter to "worker_devices".

## Problem

Currently the devices workers use is set by a `"gpu_ids"` dictionary entry in the `machine_params` method of a config file. This is a bit strange as these devices can (e.g. for debugging) be cpus.

## Solution

Let's rename `"gpu_ids"` to something more evocative (e.g. `"worker_devices"` or `"distributed_devices"`).

## Dependencies

None.

## Warnings

This will be a breaking change.