Fix unintuitive metal kernel caching (#2242)

* Fix unintuitive metal kernel caching

* alternative solution

python/tests/test_fast.py

@@ -735,6 +735,41 @@ class TestFast(mlx_tests.MLXTestCase):
         )[0]
         self.assertEqual(out.item(), 2)
 
+    @unittest.skipIf(not mx.metal.is_available(), "Metal is not available")
+    def test_custom_kernel_caching(self):
+        def call_kernel(a: mx.array, source):
+            kernel = mx.fast.metal_kernel(
+                name="my_kernel",
+                input_names=["inp"],
+                output_names=["out"],
+                source=source,
+            )
+            return kernel(
+                inputs=[a],
+                grid=(a.size, 1, 1),
+                threadgroup=(a.size, 1, 1),
+                output_shapes=[a.shape],
+                output_dtypes=[a.dtype],
+                stream=mx.gpu,
+            )[0]
+
+        a = mx.random.normal(shape=(32,))
+
+        source = """
+            uint elem = thread_position_in_grid.x;
+            out[elem] = 0.0;
+        """
+
+        out = call_kernel(a, source)
+        self.assertTrue(mx.array_equal(out, mx.zeros_like(out)))
+
+        source = """
+            uint elem = thread_position_in_grid.x;
+            out[elem] = 1.0;
+        """
+        out = call_kernel(a, source)
+        self.assertTrue(mx.array_equal(out, mx.ones_like(out)))
+
 
 if __name__ == "__main__":
     unittest.main()