Dynamic slicing (#1741)

* dynamic slice and slice update

* python bindings + tests + fix set item

* fix compile issue

* comment

* fix jit

tests/autograd_tests.cpp

@@ -1291,3 +1291,24 @@ TEST_CASE("test grad types") {
     }
   }
 }
+
+TEST_CASE("test grad dynamic slices") {
+  {
+    auto fn = [](const array& x) { return slice(x, array({0}), {0}, {1, 2}); };
+    auto x = array({1, 2, 3, 4}, {2, 2});
+    auto out = vjp(fn, x, array({1, 1}, {1, 2})).second;
+    CHECK(array_equal(out, array({1, 1, 0, 0}, {2, 2})).item<bool>());
+  }
+  {
+    auto fn = [](const std::vector<array>& inputs) {
+      const auto& x = inputs[0];
+      const auto& update = inputs[1];
+      return std::vector<array>{slice_update(x, update, array({0}), {0})};
+    };
+    auto x = zeros({2, 2});
+    auto update = array({3.f, 4.f}, {1, 2});
+    auto outs = vjp(fn, {x, update}, {ones({2, 2})}).second;
+    CHECK(allclose(outs[0], array({0.f, 0.f, 1.f, 1.f}, {2, 2})).item<bool>());
+    CHECK(allclose(outs[1], ones({1, 2})).item<bool>());
+  }
+}