Update g_partition.py

[Lamply]

bug fix

[ucalyptus2]

@Lamply @nisargshah1999 the 3 txt files are still empty even after running g_partition from @Lamply's fix.

[duguyue100]

@Lamply @nisargshah1999 the 3 txt files are still empty even after running g_partition from @Lamply's fix.

Even the original g_partition.py forget to write contents in the text files. But given you have all the train, val, and test folders, it's easy to populate these lists by walking through the folder.