[SPARK-21225][CORE] Considering CPUS_PER_TASK when allocating task slots for each WorkerOffer

JIRA Issue:https://issues.apache.org/jira/browse/SPARK-21225
    In the function "resourceOffers", It declare a variable "tasks" for storage the tasks which have allocated a executor. It declared like this:
`val tasks = shuffledOffers.map(o => new ArrayBuffer[TaskDescription](o.cores))`
    But, I think this code only conside a situation for that one task per core. If the user set "spark.task.cpus" as 2 or 3, It really don't need so much Mem. I think It can motify as follow:
val tasks = shuffledOffers.map(o => new ArrayBuffer[TaskDescription](o.cores / CPUS_PER_TASK))
 to instead.
    Motify like this the other earning is that it's more easy to understand the way how the tasks allocate offers.

Author: 杨治国10192065 <yang.zhiguo@zte.com.cn>

Closes #18435 from JackYangzg/motifyTaskCoreDisp.

core/src/main/scala/org/apache/spark/scheduler/TaskSchedulerImpl.scala

@@ -345,7 +345,7 @@ private[spark] class TaskSchedulerImpl(
 
     val shuffledOffers = shuffleOffers(filteredOffers)
     // Build a list of tasks to assign to each worker.
-    val tasks = shuffledOffers.map(o => new ArrayBuffer[TaskDescription](o.cores))
+    val tasks = shuffledOffers.map(o => new ArrayBuffer[TaskDescription](o.cores / CPUS_PER_TASK))
     val availableCpus = shuffledOffers.map(o => o.cores).toArray
     val sortedTaskSets = rootPool.getSortedTaskSetQueue
     for (taskSet <- sortedTaskSets) {