Support LR schedulers (#334)

* Add a few LR schedulers

* Move parents's constructor call to the top

* Fix docstring

* refactor optimizers into two files

* add docs

* nit

* Fix Callable type annotation for python 3.8

---------

Co-authored-by: Awni Hannun <awni@apple.com>
Co-authored-by: Angelos Katharopoulos <a_katharopoulos@apple.com>

python/src/array.cpp

@@ -971,6 +971,12 @@ void init_array(py::module_& m) {
             return power(a, to_array(v, a.dtype()));
           },
           "other"_a)
+      .def(
+          "__rpow__",
+          [](const array& a, const ScalarOrArray v) {
+            return power(to_array(v, a.dtype()), a);
+          },
+          "other"_a)
       .def(
           "__ipow__",
           [](array& a, const ScalarOrArray v) {