Limit colocation group devices after a node is assigned

This change makes Placer's code less brittle. A quick recap of Placer:
Placer first groups nodes into colocation groups. Each group is a set of
nodes and a set of possible devices that can be used for the nodes in
the group. The set of possible devices for a group is encoded in the root
Member of the tree representing this group. All the nodes in a group must
be assigned to the same device.

Before this change, the "all nodes in a group must be assigned to the same
device" property was ensured by a relying on specifics of GetDevicesForNode
and Placer's heuristics.

This change explicitly limits the possible set of devices to the single device
that a node in the group was assigned to. All future calls to GetDevicesForNode
will return just this device making it harder for a heuristic to choose
another device.

I did not find an easy way to test this change because there is no behavior
change. My future changes include a test for this that utilizes multi-device
function calls.

PiperOrigin-RevId: 231628964