Separate out the Runner's behaviour into a "connect" phase and a "load" phase

[Evanev7]

Motivation

We should ensure all runners are connected before loading the model - this gives us finer grained control in the future for the workers planning mechanism over the runners state.

Changes

• Introduced task ConnectToGroup, preceeding LoadModel
• Introduced runner statuses Idle, Connecting, Connected
• Separated out initialize_mlx from shard_and_load
• Single instances never go through the connecting phase

Test Plan

None right now.