[MIGraphX EP Support] Update migx scripts (#21806)
### Description
<!-- Describe your changes. -->
No code changes to the EP only changes to the scripts whihc invoke
MIGraphX EP
- One case be explicit to set MIGraphX EP when running gpt2 testing
- The other to ensure we turn off optimizations like tensorRT and allow
MIGraphX to handle graph optimizations
### Motivation and Context
<!-- - Why is this change required? What problem does it solve?
- If it fixes an open issue, please link to the issue here. -->
MIGraphX has moved away from using rocBLAS and without this, some cases
used in CI shall fail as optmizations will attempt to use rocBLAS
kernels instead of MIGraphx EP directly.
