Only complain about threads once (#816)

This cuts down on logging spam.

multilinear_extensions/src/util.rs

@@ -40,11 +40,16 @@ pub fn max_usable_threads() -> usize {
     if cfg!(test) {
         1
     } else {
-        let n = rayon::current_num_threads();
-        let threads = prev_power_of_two(n);
-        if n != threads {
-            tracing::warn!("thread size {n} is not power of 2, using {threads} threads instead.");
-        }
-        threads
+        static MAX_USABLE_THREADS: std::sync::OnceLock<usize> = std::sync::OnceLock::new();
+        *MAX_USABLE_THREADS.get_or_init(|| {
+            let n = rayon::current_num_threads();
+            let threads = prev_power_of_two(n);
+            if n != threads {
+                tracing::warn!(
+                    "thread size {n} is not power of 2, using {threads} threads instead."
+                );
+            }
+            threads
+        })
     }
 }