Fix bug in Collector with Graph data (#456)

* Fix bug in Collector with Graph data
* Add comments in DataModule class and bug fix in collate

pina/collector.py

@@ -1,3 +1,7 @@
+"""
+# TODO
+"""
+from .graph import Graph
 from .utils import check_consistency
 
 
@@ -52,6 +56,8 @@ class Collector:
                 # get data
                 keys = condition.__slots__
                 values = [getattr(condition, name) for name in keys]
+                values = [value.data if isinstance(
+                    value, Graph) else value for value in values]
                 self.data_collections[condition_name] = dict(zip(keys, values))
                 # condition now is ready
                 self._is_conditions_ready[condition_name] = True