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>

docs/src/python/optimizers/optimizer.rst

@@ -0,0 +1,23 @@
+Optimizer
+=========
+
+.. currentmodule:: mlx.optimizers
+
+.. autoclass:: Optimizer 
+
+
+   .. rubric:: Attributes
+
+   .. autosummary::
+      :toctree: _autosummary
+
+      Optimizer.state
+   
+   .. rubric:: Methods
+
+   .. autosummary::
+      :toctree: _autosummary
+   
+      Optimizer.apply_gradients
+      Optimizer.init
+      Optimizer.update