onnxruntime

Support two stages onnx GPT-2 conversion (#14025)

### Description

Add support of ONNX conversion of GPT-2 for two stages:
* Stage 1 is the initial stage that has empty past state. 
* Stage 2 has non-empty past state and sequence_length is 1.

Add a parameter --stage to specify such stage. For stage 1, we will
enable mask_index for Attention so that we can use fused attention in
CUDA.

Other changes:
(1) use int32 inputs as default (otherwise, there is error in inference)
(2) update gpt2_parity to include SkipLayerNormalization (see
https://github.com/microsoft/onnxruntime/pull/13988) and
EmbedLayerNormalization
(3) get all environment variables that might impact GPT-2 latency in
benchmark_gpt2

### Motivation and Context

To test fused attention for GPT-2 model for
https://github.com/microsoft/onnxruntime/pull/13953.