Add Bert Optimization Notebooks (#3204)
* Add notebooks for GPU and CPU inference of PyTorch BERT SQuAD model
* update bert_optimization.py: Do not add duplicated logger handler
* Add machineinfo.py to show machine configuration for notebook.
* Update bert performance test tool:
(1) Set OpenMP environment variable before importing onnxruntime.
(2) Use sub-process for each test
(3) Allow test multiple batch_size
(4) Add latency percentile
(5) Add warmup
