llm improvements

- document the tokenizer used (https://github.com/huggingface/swift-transformers)
- provide a hook for tokenizer configuration, prompt augmentation
	- this isn't as rich as the python equivalents but it helps a little

mlx-swift-examples.xcodeproj/project.xcworkspace/xcshareddata/swiftpm/Package.resolved

@@ -15,7 +15,7 @@
       "location" : "https://github.com/ml-explore/mlx-swift",
       "state" : {
         "branch" : "main",
-        "revision" : "fbe215ae29ec286bdc66f4b3423e3eea1b3ef2fc"
+        "revision" : "83bd19f9fe93d77d9f89981eeead2d6d190afdba"
       }
     },
     {