Use rindex instead of index for split_idx

[yahskapar]

As the title suggests. I think this makes more sense than how it currently is based on how input files get saved in the first place. rindex basically finds the last occurrence of a specified value. Since input{chunk_id} is guaranteed to be at the end of the input filename, rindex makes more sense to use in case a passed in item_pathfilename ever had more than one `in it andindexreturned an erroneoussplitidxbecause it just happened to find thein something likesubject_c`.

[yahskapar]

Tested with PURE_UNSUPERVED.yaml and everything appears to be ok. Also verified that printing out certain variables in the BaseLoader made sense.

[yahskapar]

@xliucs updated the commit with some comments that are a bit verbose, but hopefully they're helpful.