onnxruntime

Add GPT2 Attention Fusion in optimization script (#3488)

* Add Attention fusion for GPT2
* Support distilgpt2 in benchmark_gpt2.py
* Add options to disable Attention/SkipLayerNormalization/EmbedLayerNormalization/BiasGelu fusions
* Add logging at the begining of each fusion
* Update notebooks: Add Gpt2OnnxModel.py to list of script files.
* Add test for gpt2 model optimization
* Add optional parameters (--input_ids --segment_ids --input_mask) for graph inputs
* Fuse BiasGelu
* Handle model that does not have segment_ids input.
* Allow fuse embed layer without mask