[SPARK-4477] [PySpark] remove numpy from RDDSampler

In RDDSampler, it try use numpy to gain better performance for possion(), but the number of call of random() is only (1+faction) * N in the pure python implementation of possion(), so there is no much performance gain from numpy.

numpy is not a dependent of pyspark, so it maybe introduce some problem, such as there is no numpy installed in slaves, but only installed master, as reported in SPARK-927.

It also complicate the code a lot, so we may should remove numpy from RDDSampler.

I also did some benchmark to verify that:
```
>>> from pyspark.mllib.random import RandomRDDs
>>> rdd = RandomRDDs.uniformRDD(sc, 1 << 20, 1).cache()
>>> rdd.count()  # cache it
>>> rdd.sample(True, 0.9).count()    # measure this line
```
the results:

|withReplacement      |  random  | numpy.random |
 ------- | ------------ |  -------
|True | 1.5 s|  1.4 s|
|False|  0.6 s | 0.8 s|

closes #2313

Note: this patch including some commits that not mirrored to github, it will be OK after it catches up.

Author: Davies Liu <davies@databricks.com>
Author: Xiangrui Meng <meng@databricks.com>

Closes #3351 from davies/numpy and squashes the following commits:

5c438d7 [Davies Liu] fix comment
c5b9252 [Davies Liu] Merge pull request #1 from mengxr/SPARK-4477
98eb31b [Xiangrui Meng] make poisson sampling slightly faster
ee17d78 [Davies Liu] remove = for float
13f7b05 [Davies Liu] Merge branch 'master' of http://git-wip-us.apache.org/repos/asf/spark into numpy
f583023 [Davies Liu] fix tests
51649f5 [Davies Liu] remove numpy in RDDSampler
78bf997 [Davies Liu] fix tests, do not use numpy in randomSplit, no performance gain
f5fdf63 [Davies Liu] fix bug with int in weights
4dfa2cd [Davies Liu] refactor
f866bcf [Davies Liu] remove unneeded change
c7a2007 [Davies Liu] switch to python implementation
95a48ac [Davies Liu] Merge branch 'master' of github.com:apache/spark into randomSplit
0d9b256 [Davies Liu] refactor
1715ee3 [Davies Liu] address comments
41fce54 [Davies Liu] randomSplit()

1 files changed, 6 insertions, 4 deletions

diff --git a/python/pyspark/rdd.py b/python/pyspark/rdd.py
index 50535d2711..57754776fa 100644
--- a/python/pyspark/rdd.py
+++ b/python/pyspark/rdd.py
@@ -310,8 +310,11 @@ class RDD(object):
 
     def sample(self, withReplacement, fraction, seed=None):
         """
-        Return a sampled subset of this RDD (relies on numpy and falls back
-        on default random generator if numpy is unavailable).
+        Return a sampled subset of this RDD.
+
+        >>> rdd = sc.parallelize(range(100), 4)
+        >>> rdd.sample(False, 0.1, 81).count()
+        10
         """
         assert fraction >= 0.0, "Negative fraction value: %s" % fraction
         return self.mapPartitionsWithIndex(RDDSampler(withReplacement, fraction, seed).func, True)
@@ -343,8 +346,7 @@ class RDD(object):
     # this is ported from scala/spark/RDD.scala
     def takeSample(self, withReplacement, num, seed=None):
         """
-        Return a fixed-size sampled subset of this RDD (currently requires
-        numpy).
+        Return a fixed-size sampled subset of this RDD.
 
         >>> rdd = sc.parallelize(range(0, 10))
         >>> len(rdd.takeSample(True, 20, 1))