Sets the profiler backend to nvtx for improved performance in xr applications. (#3255)

# Description
Performance overhead varies based on the profiler backend even when
profiling is not enabled. In limited testing with AR profile enabled,
this yields about 5% improvement in framerate.

Follow up of #3238

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apps/isaaclab.python.xr.openxr.kit

@@ -49,6 +49,8 @@ cameras_enabled = true
 
 [settings]
 app.xr.enabled = true
+# Set profiler backend to NVTX by default
+app.profilerBackend = "nvtx"
 
 # xr settings
 xr.ui.enabled = false