[examples] fix no grad in second pruning in run_bertology (#4479)
[examples] fix no grad in second pruning in run_bertology (#4479)

* fix no grad in second pruning and typo

* fix prune heads attention mismatch problem

* fix

* fix

* fix

* run make style

* run make style
huggingface/transformers,huggingface/pytorch-transformers,huggingface/transformers,huggingface/transformers,huggingface/transformers