Examples link to the master branch which seems to be ahead of the pip install version?
Documentation for training a language model points to https://github.com/huggingface/transformers/blob/master/examples/run_language_modeling.py

The import 
`from transformers import (
    CONFIG_MAPPING,
    MODEL_WITH_LM_HEAD_MAPPING,
    AutoConfig,
    AutoModelWithLMHead,
    AutoTokenizer,
    DataCollatorForLanguageModeling,
    HfArgumentParser,
    LineByLineTextDataset,
    PreTrainedTokenizer,
    TextDataset,
    Trainer,
    TrainingArguments,
    set_seed,
)`

leads to "No name ___ in module transformers" errors.

I installed transformers using pip.

`pip install -U transformers`

The import problems seem to be gone when I install from source.
I would use the version installed from source but training a tokenizer is substantially slower than the pip install version. When I run the code snippet below, the pip install version finishesin ~5 minutes but the install-from-source version takes 20+ (I didn't wait for it to finish). For some reason there's a new "Reading files" stage which takes forever to finish.

```import os
from tokenizers import ByteLevelBPETokenizer, BertWordPieceTokenizer


tokenizer = ByteLevelBPETokenizer(lowercase=False)

paths = ["./data.txt"]

tokenizer.train(files=paths, vocab_size=52000, min_frequency=2, special_tokens=[
    "<s>",
    "<pad>",
    "</s>",
    "<unk>",
    "<mask>",
])

out_path = os.path.join(os.path.dirname(os.path.realpath(__file__)), "token")
os.makedirs(out_path, exist_ok=True)

tokenizer.save(out_path)```<|||||>This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.
