Removing import WarmupLinearSchedule

[akhileshgotmare]

Not supported by latest transformers library. See issue - https://github.com/huggingface/transformers/issues/2082 If warmup is required, we can instead use scheduler = get_linear_schedule_with_warmup(optimizer, num_warmup_steps=WARMUP_STEPS, num_training_steps = -1) instead of scheduler = WarmupLinearSchedule(optimizer, warmup_steps=WARMUP_STEPS, t_total = -1)