Update optimize_pipeline for SDXL (#17536)
- [x] Optimize SDXL models exported by optimum.
- [x] Enable it to run locally instead of using module.
- [x] Detect external data file in original model, and save with same
format by default.
- [x] Add tests
### Example
```
pip install optimum transformers diffusers onnx onnxruntime-gpu>=1.16
optimum-cli export onnx --model stabilityai/stable-diffusion-xl-base-1.0 --task stable-diffusion-xl ./sd_xl_base_onnx
python -m onnxruntime.transformers.models.stable_diffusion.optimize_pipeline -i ./sd_xl_base_onnx -o ./sd_xl_base_fp16 --float16
```
### Known issues
(1) VAE decoder cannot be converted to float16. Otherwise, there will be
black image in output.
(2) To use the float16 models, need a minor change in optimum to convert
the inputs for VAE decoder from float16 to float32 since we keep VAE
decoder as float32. The change is to append a line like the following
after [this
line](https://github.com/huggingface/optimum/blob/afd2b5a36663bebd1f501486acee065c728947bc/optimum/pipelines/diffusers/pipeline_stable_diffusion_xl.py#L483)
```
latents = latents.astype(np.float32)
```
