[TRT RTX EP] Don't register set device function when we use existing stream (#26542)
### Description
- Don't register set device function when we use existing stream
- Fix bug nv_execution_provider.cc : set device only if user did not
provide existing stream
### Motivation and Context
In some use cases, we push a user generated CUDA context, create streams
using this context, and then provide these streams to TRT-RTX.
However, we noticed that after calling Run(), the custom context is
replaced by another CUDA context created by ORT. This means that TRT-RTX
is no longer using the original CUDA context.
After investigating further, we found that the new context is being
created in onnxruntime/core/framework/stream_execution_context.cc.
The solution we propose is to not register set device function if we
provide the stream.
Also there is a bug in
onnxruntime\core\providers\nv_tensorrt_rtx\nv_execution_provider.cc.
We should set the device only if the user has not provided any stream.
(coherent with the original comment)