Fast Hadamard Transform (#1249)

* Working hadamard for powers of 2

* working for m*2^k

* add scale and check contiguity

* add size check

* clean up

* fix test

* add grads + vmap

* gpu only

* skip on linux

* test typo

* add cpu impl

* remove gpu only tests

* fix linux build + add is_equivalent

mlx/ops.cpp

@@ -451,6 +451,18 @@ array flatten(const array& a, StreamOrDevice s /* = {} */) {
   return flatten(a, 0, a.ndim() - 1, s);
 }
 
+array hadamard_transform(
+    const array& a,
+    float scale /* = 1.0 */,
+    StreamOrDevice s /* = {} */) {
+  auto dtype = issubdtype(a.dtype(), floating) ? a.dtype() : float32;
+  return array(
+      a.shape(),
+      dtype,
+      std::make_shared<Hadamard>(to_stream(s), scale),
+      {astype(a, dtype, s)});
+}
+
 array squeeze(
     const array& a,
     const std::vector<int>& axes,