Support more model outputs for BERT/ERNIE/RoBERTa (#2665)

* Support more model outputs for BERT.

* Make more task classed support more outputs.

* Convert list to tuple automatically in ModelOutput.

* Support more model outputs for ERNIE.

* Support more model outputs for Roberta.

* Add embedding output to all hidden states.

* Clean import in model_output.py

* Make output of BERT/ERNIE/RoBerta be tensor instead of tuple when the size of tuple is one.

* Make monkey patches for forward of PretrainedModel compatible with the latest forward.

* Add compatibility for patches of paddle.nn.Transformer

* Fix dygraph to static graph with more outputs.

* Fix setattr for subclass of TransformerEncoder such as Electra.

paddlenlp/transformers/__init__.py

@@ -122,4 +122,4 @@
 if is_faster_tokenizer_available():
     from .bert.faster_tokenizer import *
     from .ernie.faster_tokenizer import *
-    from .tinybert.faster_tokenizer import *
+    from .tinybert.faster_tokenizer import *