mletrasdl/pd12m-subset

Add PD12M_part2 subset: train=250000, validation=10000

Add PD12M_part2 subset: train=250000, validation=10000 (part 00001-of-00002)

Add PD12M_part2 subset: train=250000, validation=10000 (part 00000-of-00002)

initial commit