Use num-cpus to enable parallelism.

candle-core/src/utils.rs

@@ -0,0 +1,12 @@
+use std::str::FromStr;
+
+pub(crate) fn get_num_threads() -> usize {
+    // Respond to the same environment variable as rayon.
+    match std::env::var("RAYON_NUM_THREADS")
+        .ok()
+        .and_then(|s| usize::from_str(&s).ok())
+    {
+        Some(x) if x > 0 => x,
+        Some(_) | None => num_cpus::get(),
+    }
+}