warmup for BertAdam
https://github.com/huggingface/pytorch-pretrained-BERT/blob/ee0308f79ded65dac82c53dfb03e9ff7f06aeee4/examples/run_classifier.py#L860

BertAdam() can update learning rate by itself.
 Why update learning rate manually here?
Because we don't use BertAdam in fp16 mode but the optimizer of NVIDIA's apex library.<|||||>OK thank you!