Add isnan (#423)

mlx/ops.cpp

@@ -1050,6 +1050,10 @@ array array_equal(
   }
 }
 
+array isnan(const array& a, StreamOrDevice s /* = {} */) {
+  return not_equal(a, a, s);
+}
+
 array where(
     const array& condition,
     const array& x,

mlx/ops.h

@@ -374,6 +374,8 @@ array_equal(const array& a, const array& b, StreamOrDevice s = {}) {
   return array_equal(a, b, false, s);
 }
 
+array isnan(const array& a, StreamOrDevice s = {});
+
 /** Select from x or y depending on condition. */
 array where(
     const array& condition,