Usd Reader should store the unresolved "over" assignments

[sebastienblor]

When a usd file has "over" assignments on nodes that don't exist in the usd stage, this information is lost when converted to arnold in the usd reader. These informations should be stored somehow, as metadata, operators, or something else, as long as they're not lost, and can eventually be converted back to usd with the usd writer

[sebastienblor]

So, after more investigation, the reader doesn't see the unresolved "over" assignments as it uses UsdStage::Traverse. If we use instead UsdStage::TraverseAll then we see these statements. However, this might make it slower to traverse a file, it needs to be investigated. We might want to make it an attribute in the procedural, which determines how the stage is traversed.

[autodesk-oss-arnold-bot[bot]]

Issue synced internally to ARNOLD-13257