fix: Allow building only llamacpp or only mistralrs engine. (#1328)

This allows building:
-  only `mistral.rs` engine: `--no-default-features --features mistralrs`  
- or only `llama.cpp` engine: `--no-default-features --features llamacpp`. 

Since llama.cpp became a default we'd only tested building both at once. The docs already said we supported that but there was some combo of Rust features that didn't build. This is the fix.

launch/dynamo-run/src/lib.rs

@@ -116,9 +116,9 @@ pub async fn run(
 
     let out_opt = out_opt.unwrap_or_else(|| {
         let default_engine = if card.is_gguf() {
-            Output::LlamaCpp
+            gguf_default()
         } else {
-            Output::MistralRs
+            safetensors_default()
         };
         tracing::info!(
             "Using default engine: {default_engine}. Use out=<engine> to specify one of {}",
@@ -400,3 +400,22 @@ fn print_cuda(output: &Output) {
 
 #[cfg(not(any(feature = "mistralrs", feature = "llamacpp")))]
 fn print_cuda(_output: &Output) {}
+
+fn gguf_default() -> Output {
+    #[cfg(feature = "llamacpp")]
+    return Output::LlamaCpp;
+
+    #[cfg(all(feature = "mistralrs", not(feature = "llamacpp")))]
+    return Output::MistralRs;
+
+    #[cfg(not(any(feature = "mistralrs", feature = "llamacpp")))]
+    return Output::EchoFull;
+}
+
+fn safetensors_default() -> Output {
+    #[cfg(feature = "mistralrs")]
+    return Output::MistralRs;
+
+    #[cfg(not(feature = "mistralrs"))]
+    return Output::EchoFull;
+}