[code refine] if only one candidate node, no need do scoring for it

[wpeng102]

In allocate action, if only get one candidate node after PredicateNodes, no need to socreing for that node. scheduler can use this node derectly.

Just like following kubernetest scheduler logic:
https://github.com/kubernetes/kubernetes/blob/2cb3c7f706dbf266820fbde2e1b23a320e5d3de7/pkg/scheduler/schedule_one.go#L338-L347

[k82cn]

LGTM overall; how about switch for this PR? Something like that:

switch {
	case len(candidateNodes) == 0: // If not candidate nodes for this task, skip it.
		continue
	case len(candidateNodes) == 1: // If only one node after predicate, just use it.
		node = candidateNodes[0]
	case len(candidateNodes) > 1: // If more than one node after predicate, using "the best" one
                nodeScores := util.PrioritizeNodes(task, candidateNodes, ssn.BatchNodeOrderFn, ssn.NodeOrderMapFn, ssn.NodeOrderReduceFn)

		node := ssn.BestNodeFn(task, nodeScores)
		if node == nil {
			node = util.SelectBestNode(nodeScores)
			node := ssn.BestNodeFn(task, nodeScores)
			if node == nil {
				node = util.SelectBestNode(nodeScores)
			}
		}
}

[k82cn]

/approve

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[k82cn]

/lgtm