[feature] Add a option for launching the ray-head with num_cpus=0

[valayDave]

From Ray Docs

Ray resources are logical and don’t need to have 1-to-1 mapping with physical resources. For example, you can start a Ray head node with 0 logical CPUs via ray start --head --num-cpus=0 even if it physically has eight (This signals the Ray scheduler to not schedule any tasks or actors that require logical CPU resources on the head node, mainly to reserve the head node for running Ray system processes.). They are mainly used for admission control during scheduling.

for very large gang sizes, it may become important to ensure that the control task runs only the "user code" and the "head node", and the ray head node schedules no tasks on the "control node" (Node which has the UBF_CONTEXT == "control").

• https://docs.ray.io/en/latest/cluster/vms/user-guides/large-cluster-best-practices.html#configuring-the-head-node