Move postproc tests to dedicated file (meta-pytorch#3383)

Summary:
Pull Request resolved: meta-pytorch#3383

Add a dedicated file for testing postproc modules of train pipelines.
* Moved the original tests from `test_train_pipelines.py` to newly created `test_postproc.py`
* Fixed a couple of original tests where duplicated feature names are complained by KJT validator. Those extra input for postproc modules should have different feature names.
* Tests for postproc in `_rewrite_model()` will be added later

Reviewed By: TroyGarden

Differential Revision: D82688633

fbshipit-source-id: f322edddaf56357d39be5b3fbae50fa0f7a04e6b

Author: Shuangping Liu