Develop
Fixing `run_squad.py` pre-processing bug.

Various clean-ups:

- the weight initialization was not optimal (tf. truncated_normal_initializer(stddev=0.02) was translated in weight.data.normal_(0.02) instead of weight.data.normal_(mean=0.0, std=0.02) which likely affected the performance of run_classifer.py also.
- gradient accumulation loss was not averaged over the accumulation steps which would have required to change the hyper-parameters for using accumulation.
- the evaluation was not done with torch.no_grad() and thus sub-optimal in terms of speed/memory.
