Fix a bug in combining chunks in a table (pytorch#122)

Summary:
Pull Request resolved: pytorch#122

`combine_chunks()` is const. Forgot to update the handle with the returned from `combine_chunks()`

Reviewed By: vancexu

Differential Revision: D33234089

fbshipit-source-id: b72069ba513098cb6284f1e04d1b25675398ba0f

Support Python 3.7 (pytorch#93)

Summary:
Google Collab runs Python 3.7

Pull Request resolved: pytorch#93

Reviewed By: ejguan

Differential Revision: D32734745

Pulled By: wenleix

fbshipit-source-id: 7d187c8ccdca7a59668c925c0712d59f7d27d79d

Add transformation-only test that works for multiple runtime (pytorch#84

)

Summary:
Pull Request resolved: pytorch#84

This allows write unified test across eager and lazy (or tracing-based) runtime.

Some runtime may only support transformation across columns from the same dataframe, thus the existing eager mode tests (which creates Column and test them) doesn't work.

The idea is test data is stored in base test class as a CPU dataframe.  And the concrete backend test will convert the CPU dataframe to backend specific dataframe for testing (e.g. GPU or even lazy df) and assert the dataframe transformation has the same behavior.

Python doesn't have native "abstract base test" support. So use this `Raise unittest.SkipTest` trick: https://stackoverflow.com/a/59561905.

Another approach is similar to existing TorchArrow test setup, have `base_test_X` and let subclass to override as `test_X`.

Reviewed By: bhuang3

Differential Revision: D32337291

fbshipit-source-id: 871ccd85891e37e52b2bfbaa9c901c7b265190ac