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>

Tools/llm-tool/Arguments.swift

@@ -0,0 +1,15 @@
+// Copyright © 2024 Apple Inc.
+
+import ArgumentParser
+import Foundation
+
+/// Extension to allow URL command line arguments.
+extension URL: ExpressibleByArgument {
+    public init?(argument: String) {
+        if argument.contains("://") {
+            self.init(string: argument)
+        } else {
+            self.init(filePath: argument)
+        }
+    }
+}