[FLINK-3993] [py] Add generateSequence() support to Python API

[omaralvarez]

Addition of generate_sequence() to python API:

data = env.generate_sequence(1,20)

If a parallel number source was needed, there was no option to create it, since from_elements() is not parallel.

[zentol]

Thank you for your contribution. Overall the code is correct, but i would prefer if you would the various "frm" fields to "from".

Also, a small update to the documentation, and a simple test would be appreciated.

[omaralvarez]

About the "frm", in Python when using from, there is a syntax error since from is a system reserved word. In the Java side of things there is no problem with renaming and using from, but I chose to use also frm to maintain consistency.

I will get right on the documentation and test. Should I add commits or squash them in the one commit?

[zentol]

please add them as a separate commit, they will be squashed when merging.

Good point regarding frm.

[omaralvarez]

I think, everything should be ready now. Although while performing testing, I have found something that I think is not ideal.

If we use this code:

env.generate_sequence(1, 5)\
         .map(Id()).map_partition(Verify([1,2,3,4], "Sequence")).output()

The Verify() function will error out with IndexError: list index out of range, this is not ideal, since it should raise a Flink testing exception. I could also try to fix this if needed.

[zentol]

Changing the test infrastructure should be done as a separate issue. There are a few things that could be improved, especially a bit of documentation (like what is the difference between Verify and Verify2).

Your changes look good, I'll merge them later today.

[omaralvarez]

Perfect. I'll open a new issue and try to fix that and document the test scripts.