[SPARK-15150][EXAMPLE][DOC] Update LDA examples

[zhengruifeng]

What changes were proposed in this pull request?

1,create a libsvm-type dataset for lda: data/mllib/sample_lda_libsvm_data.txt
2,add python example
3,directly read the datafile in examples
4,BTW, change to SparkSession in aft_survival_regression.py

How was this patch tested?

manual tests
./bin/spark-submit examples/src/main/python/ml/lda_example.py

[SparkQA]

Test build #57865 has finished for PR 12927 at commit b98ad7c.

• This patch passes all tests.
• This patch merges cleanly.
• This patch adds no public classes.

[SparkQA]

Test build #57867 has finished for PR 12927 at commit 543df2f.

• This patch passes all tests.
• This patch merges cleanly.
• This patch adds no public classes.

[HyukjinKwon]

@dongjoon-hyun Sorry for cc a lot but it would be great if I can hear your thoughts. I saw your PRs fixing some examples with the component [EXAMPLE] in the title.

[SparkQA]

Test build #57968 has finished for PR 12927 at commit 2270148.

• This patch passes all tests.
• This patch merges cleanly.
• This patch adds no public classes.

[yanboliang]

println(s"The lower bound on the log likelihood of the entire corpus: $ll")
println(s"The upper bound bound on perplexity: $lp")

[SparkQA]

Test build #57991 has finished for PR 12927 at commit d7916b4.

• This patch passes all tests.
• This patch merges cleanly.
• This patch adds no public classes.

[SparkQA]

Test build #58088 has finished for PR 12927 at commit 8625754.

• This patch passes all tests.
• This patch merges cleanly.
• This patch adds no public classes.

[SparkQA]

Test build #58089 has finished for PR 12927 at commit 0191d6a.

• This patch passes all tests.
• This patch merges cleanly.
• This patch adds no public classes.

[SparkQA]

Test build #58101 has finished for PR 12927 at commit 5e26019.

• This patch passes all tests.
• This patch merges cleanly.
• This patch adds no public classes.

[SparkQA]

Test build #58102 has finished for PR 12927 at commit 1219ca1.

• This patch passes all tests.
• This patch merges cleanly.
• This patch adds no public classes.

[SparkQA]

Test build #58316 has finished for PR 12927 at commit 65dd7ca.

• This patch passes all tests.
• This patch merges cleanly.
• This patch adds no public classes.

[SparkQA]

Test build #58317 has finished for PR 12927 at commit eab9511.

• This patch passes all tests.
• This patch merges cleanly.
• This patch adds no public classes.

[SparkQA]

Test build #58329 has finished for PR 12927 at commit eab9511.

• This patch passes all tests.
• This patch merges cleanly.
• This patch adds no public classes.

[MLnick]

Describe topics

[MLnick]

minor comment on some comments, otherwise LGTM. @yanboliang ?

[SparkQA]

Test build #58345 has finished for PR 12927 at commit f573fa6.

• This patch passes all tests.
• This patch merges cleanly.
• This patch adds no public classes.

[MLnick]

Please rebase your PR to current master and fix merge conflicts

[SparkQA]

Test build #58356 has finished for PR 12927 at commit 83918d4.

• This patch passes all tests.
• This patch merges cleanly.
• This patch adds no public classes.

[yanboliang]

LGTM. Thanks!

[MLnick]

Merged to master and branch-2.0. Thanks!