updating docs - adding few tests to tokenizers

docs/source/main_classes/model.rst

@@ -1,6 +1,13 @@
 Models
 ----------------------------------------------------
 
+The base class ``PreTrainedModel`` implements the common methods for loading/saving a model either from a local file or directory, or from a pretrained model configuration provided by the library (downloaded from HuggingFace's AWS S3 repository).
+
+``PreTrainedModel`` also implements a few methods which are common among all the models to:
+
+- resize the input token embeddings when new tokens are added to the vocabulary
+- prune the attention heads of the model.
+
 ``PreTrainedModel``
 ~~~~~~~~~~~~~~~~~~~~~