Enabled Dynamo exporter (#21713)
### Description
This PR modifies the run_dynamo_export function to ensure it mirrors the
behavior of run_torchscript_merged_export rather than
run_torchscript_separate_export. Additionally, I made adjustments to the
main function to ensure that run_dynamo is correctly invoked.
### Motivation and Context
The main motivation for this change is to enable successful export of
LLaMA-2 and LLaMA-3 models using the Dynamo exporter to ONNX.
Previously, the exporter was saving two copies of the weights, which is
inefficient. The modified approach ensures that only one copy of the
weights is saved, and the model can support both scenarios. These
changes enhance the compatibility of the exporter with LLaMA models and
subsequently other models and optimize the export process
