[examples] Generate argparsers from type hints on dataclasses (#3669)

* [examples] Generate argparsers from type hints on dataclasses

* [HfArgumentParser] way simpler API

* Restore run_language_modeling.py for easier diff

* [HfArgumentParser] final tweaks from code review

src/transformers/__init__.py

@@ -88,6 +88,7 @@ from .file_utils import (
     is_tf_available,
     is_torch_available,
 )
+from .hf_argparser import HfArgumentParser
 
 # Model Cards
 from .modelcard import ModelCard
@@ -141,6 +142,7 @@ from .tokenization_utils import PreTrainedTokenizer
 from .tokenization_xlm import XLMTokenizer
 from .tokenization_xlm_roberta import XLMRobertaTokenizer
 from .tokenization_xlnet import SPIECE_UNDERLINE, XLNetTokenizer
+from .training_args import TrainingArguments
 
 
 logger = logging.getLogger(__name__)  # pylint: disable=invalid-name