Removing cuda default.

Seems very important for a lot of exploring users usually on laptop
without GPUs.

Adding more README instructions in a follow up.

candle-examples/examples/llama/main.rs

@@ -133,10 +133,20 @@ fn main() -> Result<()> {
     use tokenizers::Tokenizer;
 
     let args = Args::parse();
+
+    #[cfg(feature = "cuda")]
+    let default_device = Device::new_cuda(0)?;
+
+    #[cfg(not(feature = "cuda"))]
+    let default_device = {
+        println!("Running on CPU, to run on GPU, run this example with `--features cuda`");
+        Device::Cpu
+    };
+
     let device = if args.cpu {
         Device::Cpu
     } else {
-        Device::new_cuda(0)?
+        default_device
     };
     let config = Config::config_7b();
     let cache = model::Cache::new(!args.no_kv_cache, &config, &device);