Fix a path problem in onnxruntime_perf_test (#21341)
### Description
Resolve #21267 . onnxruntime_perf_test does not work properly if the
input model path url is just a single filename without any path
separator. For example,
```
./onnxruntime_perf_test -t 10 model.onnx
```
The problem was introduced in #19196 by me.
