Fixing issue "Training beyond specified 't_total' steps with schedule 'warmup_linear'" reported in #556
Fixing the issues reported in https://github.com/huggingface/pytorch-pretrained-BERT/issues/556

Reason for issue was that num_optimzation_steps was computed from example size, which is different from actual size of dataloader when an example is chunked into multiple instances.

Solution in this pull request is to compute num_optimization_steps directly from len(data_loader).
Looks good.<|||||>Ok merging thanks, sorry for the delay!