Add info about property wrappers to porting guide (#347)

* Add info about property wrappers to porting guide
* Computed vs Loaded Parameters

Author: DePasqualeOrg

Libraries/MLXLMCommon/Documentation.docc/porting.md

@@ -112,6 +112,133 @@ enum CodingKeys: String, CodingKey {
 
 This will read `rope_theta` from the JSON file if present and fall back to a default value of `10_000`.
 
+### Understanding Property Wrappers
+
+MLX Swift uses property wrappers to handle automatic registration of modules and parameters. This is different from Python MLX, which uses runtime discovery.
+
+#### Python MLX: Automatic Discovery
+
+In Python MLX, you simply assign modules and parameters as attributes, and MLX automatically discovers them:
+
+```python
+class MyModule(nn.Module):
+    def __init__(self):
+        super().__init__()
+        # Just assign - MLX auto-discovers these
+        self.linear = nn.Linear(256, 256)  # sub-module
+        self.weight = mx.ones([256])       # parameter
+```
+
+#### Swift MLX: Property Wrappers
+
+In Swift MLX, you must explicitly declare modules and parameters using property wrappers:
+
+```swift
+class MyModule: Module {
+    @ModuleInfo var linear: Linear     // sub-module
+    @ParameterInfo var weight: MLXArray // parameter
+    
+    init() {
+        self._linear.wrappedValue = Linear(256, 256)
+        self._weight.wrappedValue = MLXArray.ones([256])
+        super.init()
+    }
+}
+```
+
+#### When to Use Each
+
+- **`@ModuleInfo`**: For neural network layers (anything with `callAsFunction`)
+  - Examples: `Linear`, `Attention`, `MLP`, `RMSNorm`, arrays of layers
+- **`@ParameterInfo`**: For raw `MLXArray` tensors you use manually
+  - Examples: weight matrices, bias vectors, scaling factors
+
+#### Weight Key Mapping
+
+Both property wrappers support custom keys for weight loading:
+
+```swift
+@ModuleInfo(key: "self_attn") var attention: Attention
+@ParameterInfo(key: "custom_weight") var weight: MLXArray
+```
+
+The `key` parameter maps to the corresponding key in the saved model weights. If no key is specified, it uses the property name.
+
+#### Initialization Syntax
+
+When using property wrappers, initialization uses the `wrappedValue` syntax:
+
+```swift
+// Without property wrapper:
+self.weight = MLXArray.ones([dimensions])
+
+// With property wrapper:
+self._weight.wrappedValue = MLXArray.ones([dimensions])
+```
+
+This explicit registration system provides type safety and ensures all modules and parameters are properly tracked for operations like weight loading, quantization, and gradient computation.
+
+#### Advanced Property Wrapper Patterns
+
+Beyond the basic usage, there are several advanced patterns you'll encounter:
+
+**Optional Modules**
+```swift
+@ModuleInfo(key: "lm_head") var lmHead: Linear?
+@ModuleInfo(key: "text_projection") var textProjection: Linear?
+```
+Used when modules are conditionally created based on configuration.
+
+**Arrays of Modules**
+```swift
+@ModuleInfo(key: "layers") var layers: [TransformerBlock]
+@ModuleInfo(key: "down_blocks") var downBlocks: [EncoderDecoderBlock2D]
+```
+For dynamic numbers of repeated layers.
+
+**Complex Module Types**
+```swift
+@ModuleInfo(key: "mid_blocks") var midBlocks: (ResnetBlock2D, Attention, ResnetBlock2D)
+```
+Tuples and other composite types are supported.
+
+**Optional Parameters**
+```swift
+@ParameterInfo var bias: MLXArray?
+@ModuleInfo(key: "bias") var bias: MLXArray?
+```
+For optional parameters that may not exist in all model variants.
+
+**Special Case: @ModuleInfo with MLXArray**
+```swift
+@ModuleInfo(key: "weight") var weight: MLXArray
+@ModuleInfo(key: "scales") var scales: MLXArray
+```
+In rare cases (like quantized layers), `@ModuleInfo` is used with `MLXArray` instead of `@ParameterInfo`. This typically occurs with specialized quantization or expert layers where the arrays are treated as sub-modules for weight loading purposes.
+
+**Computed vs Loaded Parameters**
+```swift
+// Parameter loaded from weights - uses @ParameterInfo
+@ParameterInfo(key: "correct_output_scale") var correctOutputScale: MLXArray
+
+// Computed parameter - uses private underscore prefix (ignored during loading)
+private let _routerInputScale: MLXArray
+```
+
+This distinction is crucial:
+- `@ParameterInfo` properties are expected to exist in the weight files and will be loaded automatically
+- Private properties with underscore prefix (`_`) are ignored during weight loading and must be computed during initialization
+- Initialization syntax differs:
+  ```swift
+  // For @ParameterInfo (loaded from weights):
+  self._correctOutputScale.wrappedValue = MLXArray(...)
+  
+  // For private computed parameters:
+  self._routerInputScale = MLXArray(...)
+  ```
+
+Use private underscore properties when you need to compute values based on configuration or other parameters, but don't want the weight loading to fail because these "parameters" don't exist in the saved weights.
+
 ### Porting Layers without Children
 
 Now we can begin porting the layers (Modules). Here is an example layer with no child layers (e.g. `Linear`) but with parameters (e.g. `MLXArray`):