UsdPrimvarReader_float2 returning "st" not working in the usd procedural

[sebastienblor]

In #1522 we have added support for UsdPrimvarReader_float2 shaders returning "st", which is working when we render a usd file.
But this is not working when the usd file is loaded through a procedural

[autodesk-oss-arnold-bot]

Issue synced internally to ARNOLD-13653