[NV TRT RTX EP] Add dimension override for TopK model in TestSessionOutputs unittest (#26975)

### Description
<!-- Describe your changes. -->

- Added dimension override in topk_and_multiple_graph_outputs model to
match K value

### 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. -->
- topk_and_multiple_graph_outputs model in TestSessionOutputs unit test
was failing due to shape constraints: TopK: length of reduction axis (N)
is smaller than K (300) Condition '==' violated: 0 != 1

Author: keshavv27

onnxruntime/test/providers/nv_tensorrt_rtx/nv_basic_test.cc

@@ -233,6 +233,9 @@ TEST(NvExecutionProviderTest, TestSessionOutputs) {
   {
     Ort::SessionOptions session_options;
     session_options.AppendExecutionProvider(kNvTensorRTRTXExecutionProvider, {});
+    // topk_and_multiple_graph_outputs.onnx contains input with dynamic dimension N
+    // setting override to avoid shape mismatch
+    session_options.AddFreeDimensionOverrideByName("N", 300);
 
     auto model_path = ORT_TSTR("testdata/topk_and_multiple_graph_outputs.onnx");
     Ort::Session session(*ort_env, model_path, session_options);