add option to load pretrained weights for T5 model (#15951)
### Description
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Adds option to pass in pretrained weights file during T5 inference onnx
export. Mimics the changes made to whisper:
https://github.com/microsoft/onnxruntime/pull/15759
### Motivation and Context
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- If it fixes an open issue, please link to the issue here. -->
Required for ONNX Runtime demo being presented at BUILD.
