fix: place ug dep behind not wasm32 flag (#2760)

* place `ug` behind not wasm32 attr

so that wasm32 can compile

* mv `ug` to conditional target dep

assuming every non-wasm32 user wants this

candle-core/src/error.rs

@@ -172,6 +172,7 @@ pub enum Error {
     #[error("Metal error {0}")]
     Metal(#[from] MetalError),
 
+    #[cfg(not(target_arch = "wasm32"))]
     #[error(transparent)]
     Ug(#[from] ug::Error),