Callbacks [wip] (#889)

* Add callback system + associated test

* Add trainer and pl_module args to callback methods

* typing

* typo in docstring

* Switch to on_.*_start()

* fix on_test_start

* fix the mess after rebasing

docs/source/callbacks.rst

@@ -9,6 +9,6 @@ Callbacks
         _save_model,
         on_epoch_end,
         on_train_end,
-        on_epoch_begin,
+        on_epoch_start,
         check_monitor_top_k,
-        on_train_begin,
+        on_train_start,

docs/source/loggers.rst

@@ -9,4 +9,4 @@ Loggers
         _save_model,
         on_epoch_end,
         on_train_end,
-        on_epoch_begin,
+        on_epoch_start,