Improve docs and fix tests around training

bindings/python/py_src/tokenizers/__init__.pyi

@@ -1022,6 +1022,45 @@ class Tokenizer:
             :obj:`Optional[int]`: An optional id, :obj:`None` if out of vocabulary
         """
         pass
+    def train(self, files, trainer=None):
+        """
+        Train the Tokenizer using the given files.
+
+        Reads the files line by line, while keeping all the whitespace, even new lines.
+        If you want to train from data store in-memory, you can check
+        :meth:`~tokenizers.Tokenizer.train_from_iterator`
+
+        Args:
+            files (:obj:`List[str]`):
+                A list of path to the files that we should use for training
+
+            trainer (:obj:`~tokenizers.trainers.Trainer`, `optional`):
+                An optional trainer that should be used to train our Model
+        """
+        pass
+    def train_from_iterator(self, iterator, trainer=None, length=None):
+        """
+        Train the Tokenizer using the provided iterator.
+
+        You can provide anything that is a Python Iterator
+
+            * A list of sequences :obj:`List[str]`
+            * A generator that yields :obj:`str` or :obj:`List[str]`
+            * A Numpy array of strings
+            * ...
+
+        Args:
+            iterator (:obj:`Iterator`):
+                Any iterator over strings or list of strings
+
+            trainer (:obj:`~tokenizers.trainers.Trainer`, `optional`):
+                An optional trainer that should be used to train our Model
+
+            length (:obj:`int`, `optional`):
+                The total number of sequences in the iterator. This is used to
+                provide meaningful progress tracking
+        """
+        pass
     @property
     def truncation(self):
         """