Refine BERT optimization script options (#3618)
* Remove paramters like --gpu_only --sequence_length. Update bert GPU notebook accordingly.
* Remove input_int32 and float16 parameters from constructors of BertOnnxModel class and other classes derived from it.
* Update gpt2 benchmark. Add comments in gpt2 notebook to indicate work in progress. Clear notebook output before official 1.3.0 release is ready.
