Simple repro steps:
1. In the Project Window, create a new subgraph operator
2. Open the subgraph operator and drag a noodle from the range property
3. Observe how outputs are allowed

Expected outcome: outputs are hidden as they serve no purpose in subgraph operators.

Notes:
- This is a simple matter of disallowing outputs to be added when a noodle is dragged (they are already disallowed in the regular node menu)
- As noticed by Akash with the repro steps below, on OSX if range is connected to a subgraph, converted to yet another output, and then the user attempts to enter the subgraph the editor freezes. This will be resolved, however, if outputs are simply not accessible in operator subgraphs.

Steps to repro:
1. Create anew HDRP template project.
2. Project Window > Right Click > Create > Visual Effects > Visual Effect Subgraph Operator.
3. Open VFX Subgraph operator > Drag range property.
4. Range > Context > Output Mesh > Right Click > Convert to Subgraph > Save.
5. Right click > Enter Subgraph.

Expected Result:
Subgraph opens up

Actual Result:
Editor freezes

Reproducible in:
2020.2.0a5 with Package Version 8.0.1
2020.1.0b4 with Package Version 8.0.1

Not Reproducible in:
2020.2.0a5 with Package Version 7.1.8
2020.1.0b4 with Package Version 7.1.8
Environment:
Occurs only on Mac