Update manifest.py to speedup loading tarred datasets
jenkins > Is there any update on this ? @stevehuang52 @anteju

@titu1994 Nope, I think it's ready to merge if there're no other comments. jenkins
A simple trick to skip checking for relative filepaths when using tarred datasets. On my local desktop, the modification reduces the manifest loading time for tarred LS data from 4.98s to 3.28s, with 34% relative speedup. The trick is ugly...so please let me know of better ways to do it...