implement LoRA / QLoRA (#46)

* implement LoRA / QLoRA

- example of using MLX to fine-tune an LLM with low rank adaptation (LoRA) for a target task
- see also https://arxiv.org/abs/2106.09685
- based on https://github.com/ml-explore/mlx-examples/tree/main/lora

* add some command line flags I found useful during use
- --quiet -- don't print decorator text, just the generated text
- --prompt @/tmp/file.txt -- load prompt from file

* user can specify path to model OR model identifier in huggingface

* update mlx-swift reference

Co-authored-by: Ashraful Islam <ashraful.meche@gmail.com>
Co-authored-by: JustinMeans <46542161+JustinMeans@users.noreply.github.com>

Libraries/LLM/Qwen2.swift

@@ -251,3 +251,11 @@ public struct Qwen2Configuration: Codable {
             [String: StringOrNumber].self, forKey: Qwen2Configuration.CodingKeys.ropeScaling)
     }
 }
+
+// MARK: - LoRA
+
+extension Qwen2Model: LoRAModel {
+    public func loraLinearLayers() -> LoRALinearLayers {
+        model.layers.map { ($0.attention, ["q_proj", "v_proj"]) }
+    }
+}