extension outside of file declaring generic struct 'AnyArrayBuffer' prevents automatic synthesis of 'move(by:)'

```
AnyArrayBuffer+Differentiable.swift:18:1: error: extension outside of file declaring generic struct 'AnyArrayBuffer' prevents automatic synthesis of 'move(by:)' for protocol 'Differentiable'
extension AnyArrayBuffer: Differentiable where Dispatch: DifferentiableArrayDispatch {
^
```

@BradLarson have you encountered this? I think this is a new rule added to the Swift compiler, but I don't know when or how.

Start with AnyArrayBuffer+Differentiable.swift at line 18 and reproduce the reported Swift compiler error around the AnyArrayBuffer extension. Check the compiler rule involved and determine the project-compatible resolution; the issue is complete when the error is understood and the project builds without it.

Written by the indexing model from the issue text.