Add trainer attribute to datamodule (#3749)

* Split out changes from #3563 to make that PR easier to review. This formats the file according to the Black formatter

* Store a reference to the trainer on the datamodule

Fixes #3682

* Update data_connector.py

* Update data_connector.py

* Update test_datamodules.py

* Add attribute to datamodule for trainer

pytorch_lightning/core/datamodule.py

@@ -147,6 +147,9 @@ def __init__(
         self._test_transforms = test_transforms
         self._dims = dims if dims is not None else ()
 
+        # Pointer to the trainer object
+        self.trainer = None
+
         # Private attrs to keep track of whether or not data hooks have been called yet
         self._has_prepared_data = False
         self._has_setup_fit = False