MaterialX: Support multi-output nodes in hdMtlx

[JGamache-autodesk]

Description of Issue

The code in _GatherUpstreamNodes() is missing support for multi-output nodes and will fail to connect them.

To detect:
Need to call isMultiOutputType on the Node. Nodes like artistic_ior should return true.

An example of using an artistic_ior node and connecting it can be found here. It calls low-level versions of the mx::Input interface to set the node name and output name. If you look at the implementations of mx::Input::setConnectedNode() and mx::Input::setConnectedOutput() you will see the same low-level calls.

The resulting MaterialX document should contain input elements that connect like usd_preview_surface:

      <input name="ior" type="color3" nodename="artistic_ior" output="ior" />

or possibliy like the dielectric Lama node:

      <input name="in2" type="float" nodename="artistic_ior" output="ior" channels="r" />

Steps to Reproduce

System Information (OS, Hardware)

Package Versions

Build Flags

[jilliene]

Filed as internal issue #USD-6820