What

• Fixed setting a value for an optional child module
• Fixed resetting an optional child module back to nil

Motivation

Consider the following example with an optional child module:

class Example: Module {
    @ModuleInfo(key: "a") var a: Linear = Linear(1, 1)
    @ModuleInfo(key: "b") var b: Linear?
}

Currently, attempting to update b via update(modules:verify:) fails:

let linear = Linear(1, 1)
let update = ModuleChildren(values: ["b": .value(linear)])
try example.update(modules: update, verify: .noUnusedKeys) // Fatal error: Unable to set b on Example(b=nil)

The reason is it has double optional value inside:

(lldb) po (value, type(of: value))
▿ 2 elements
  - .0 : nil
  - .1 : Swift.Optional<Swift.Optional<MLXNN.Linear>>

Comparing to a non-optional child module:

(lldb) po (value, type(of: value))
▿ 2 elements
  ▿ .0 : Optional<Linear>
    ▿ some : Linear(inputDimensions=1, outputDimensions=1, bias=true)
  - .1 : Swift.Optional<MLXNN.Linear>

Due to the double optional, ModuleValue.build(value:) maps the value to .other(nil) instead of .none and prevents updates from being applied correctly. This fix ensures that setting a value when the current value is nil works as expected and resetting an optional module back to nil is supported as well.