[AIMIGRAPHX-] MIGraphX ROCm 7.14 feature port. - #53

The MIGraphX EP synchronized the compute stream after every Compute call
(staging, eager, and EPContext paths), forcing a full CPU/GPU serialization
per node and destroying inference overlap - worst for graphs partitioned
into multiple MIGraphX/other nodes.

ORT owns the per-run compute stream (created in CreateSyncStreamForDevice,
handed to the kernel via GetGPUComputeStream()) and already flushes it at
run end through DeviceStreamCollection::CleanUp -> SyncStream::Flush,
honoring the run's sync_stream setting. Intra-run cross-stream ordering is
handled by ORT notifications. The EP therefore does not need to sync in the
hot path.

Also fix OnRunEnd, which synchronized the EP's default stream (stream_) -
a stream that never carried compute work, since all kernels run on ORT's
compute stream. That sync was both incorrect (wrong stream) and a redundant
serialization point, so it is dropped and the now-unused stream_ member is
removed.

Outputs remain correct: all H2D/compute/D2D work is enqueued on the compute
stream in order and ORT flushes it before returning results to the caller.