pytorch_transformers became transformers

[Famke2604]

Just wanted to let you guys know that instead of "from pytorch_transformers import ..." it is renamed to just transformers, so "from transformers import ..."

[hialoha]

besides, Huggingface has a break change for the function WarmupLinearSchedule, it's been renamed to get_linear_schedule_with_warmup and also renamed the parameters.

reference:
microsoft/nlp-recipes#509