[SPARK-21401][ML][MLLIB] add poll function for BoundedPriorityQueue

## What changes were proposed in this pull request?
The most of BoundedPriorityQueue usages in ML/MLLIB are:
Get the value of BoundedPriorityQueue, then sort it.
For example, in Word2Vec: pq.toSeq.sortBy(-_._2)
in ALS, pq.toArray.sorted()

The test results show using pq.poll is much faster than sort the value.
It is good to add the poll function for BoundedPriorityQueue.

## How was this patch tested?
The existing UT

Author: Peng <peng.meng@intel.com>
Author: Peng Meng <peng.meng@intel.com>

Closes #18620 from mpjlu/add-poll.

core/src/main/scala/org/apache/spark/util/BoundedPriorityQueue.scala

@@ -51,6 +51,10 @@ private[spark] class BoundedPriorityQueue[A](maxSize: Int)(implicit ord: Orderin
     this
   }
 
+  def poll(): A = {
+    underlying.poll()
+  }
+
   override def +=(elem1: A, elem2: A, elems: A*): this.type = {
     this += elem1 += elem2 ++= elems
   }

core/src/test/scala/org/apache/spark/util/BoundedPriorityQueueSuite.scala

@@ -0,0 +1,51 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one or more
+ * contributor license agreements.  See the NOTICE file distributed with
+ * this work for additional information regarding copyright ownership.
+ * The ASF licenses this file to You under the Apache License, Version 2.0
+ * (the "License"); you may not use this file except in compliance with
+ * the License.  You may obtain a copy of the License at
+ *
+ *    http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.apache.spark.util
+
+import org.apache.spark.SparkFunSuite
+
+class BoundedPriorityQueueSuite extends SparkFunSuite {
+  test("BoundedPriorityQueue poll test") {
+    val pq = new BoundedPriorityQueue[Double](4)
+
+    pq += 0.1
+    pq += 1.5
+    pq += 1.0
+    pq += 0.3
+    pq += 0.01
+
+    assert(pq.isEmpty == false)
+    assert(pq.poll() == 0.1)
+    assert(pq.poll() == 0.3)
+    assert(pq.poll() == 1.0)
+    assert(pq.poll() == 1.5)
+    assert(pq.isEmpty == true)
+
+    val pq2 = new BoundedPriorityQueue[(Int, Double)](4)(Ordering.by(_._2))
+    pq2 += 1 -> 0.5
+    pq2 += 5 -> 0.1
+    pq2 += 3 -> 0.3
+    pq2 += 4 -> 0.2
+    pq2 += 1 -> 0.4
+
+    assert(pq2.poll()._2 == 0.2)
+    assert(pq2.poll()._2 == 0.3)
+    assert(pq2.poll()._2 == 0.4)
+    assert(pq2.poll()._2 == 0.5)
+  }
+}