Update LLM export to use PyTorch 2.7 (#24549)
### Description
This PR updates ONNX Runtime's LLM conversion tools to use [PyTorch
2.7](https://pytorch.org/blog/pytorch-2-7/) and reduces memory usage
during export.
### Motivation and Context
Importing the `transformers` package with `import transformers` will
take a long time because of the many namespaces it has at the top level.
It is more efficient to only import the desired class names.
Additionally, the benchmarking of the PyTorch model includes the deep
copy of the inputs when it does not need to. The deep copy can be
performed before measuring latency.
