add LLM evaluator example

- runs on iOS and macOS
- downloads a model / tokenizer from hugging face
- evaluates the given prompt

Applications/LLMEval/README.md

@@ -0,0 +1,35 @@
+#  LLMEval
+
+An example that:
+
+- downloads a huggingface model (phi-2) and tokenizer
+- evaluates a prompt
+- displays the output as it generates text
+
+> Note: this _must_ be built Release, otherwise you will encounter
+stack overflows.
+
+You will need to set the Team on the LLMEval target in order to build and
+run on iOS.
+
+Some notes about the setup:
+
+- this downloads models from hugging face so LLMEval -> Signing & Capabilities has the "Outgoing Connections (Client)" set in the App Sandbox
+- LLM models are large so this uses the Increased Memory Limit entitlement on iOS to allow ... increased memory limits for devices that have more memory
+- The Phi2 4 bit model is small enough to run on some iPhone models
+
+### Troubleshooting
+
+If the program crashes with a very deep stack trace you may need to build
+in Release configuration.  This seems to depend on the size of the model.
+
+There are a couple options:
+
+- build Release
+- force the model evaluation to run on the main thread, e.g. using @MainActor
+- build `Cmlx` with optimizations by modifying `mlx/Package.swift` and adding `.unsafeOptions(["-O3"]),` around line 87
+
+Building in Release / optimizations will remove a lot of tail calls in the C++ 
+layer.  These lead to the stack overflows.
+
+See discussion here: https://github.com/ml-explore/mlx-swift-examples/issues/3