Ghost variables and function in data_loader.py

[preetom-saha-arko]

Three variables named 'ratio', 'dw' and 'dh' have been used in data_loader.py but these variables have no reference anywhere in the file.

The function named _truncate_seq_pair() has also been used in the file but hasn't been implemented anywhere. I guess you took it from https://github.com/google-research/bert/blob/master/run_classifier.py