BertForMaskedLM "labels" is an unexpected keyword
# 🐛 Bug

## Information

The official BertForMaskedLM example: https://huggingface.co/transformers/model_doc/bert.html#bertformaskedlm
 has a bug. When running: outputs = model(input_ids, labels=input_ids), it alerts: TypeError: forward() got an unexpected keyword argument 'labels'

Model I am using (Bert, XLNet ...):
BERT

Language I am using the model on (English, Chinese ...):
English

The problem arises when using:
* [ ] the official example scripts: (give details below)
from transformers import BertTokenizer, BertForMaskedLM
import torch

tokenizer = BertTokenizer.from_pretrained('bert-base-uncased')
model = BertForMaskedLM.from_pretrained('bert-base-uncased')

input_ids = torch.tensor(tokenizer.encode("Hello, my dog is cute", add_special_tokens=True)).unsqueeze(0)  # Batch size 1
outputs = model(input_ids, labels=input_ids)

* [ ] my own modified scripts: (give details below)
The tasks I am working on is:
* [ ] an official GLUE/SQUaD task: (give the name)
* [ ] my own task or dataset: (give details below)

## To reproduce

Steps to reproduce the behavior:

1. https://huggingface.co/transformers/model_doc/bert.html#bertformaskedlm
2.
3.

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## Expected behavior
should work with labels??
<!-- A clear and concise description of what you would expect to happen. -->

## Environment info
<!-- You can run the command `transformers-cli env` and copy-and-paste its output below.
     Don't forget to fill out the missing fields in that output! -->
     
- `transformers` version: 2.11.0
- Platform: Linux
- Python version: 3.7
- PyTorch version (GPU?): 1.5.1
- Tensorflow version (GPU?): 2.2.0
- Using GPU in script?: yes 
- Using distributed or parallel set-up in script?: no

Hi, @guoxuxu , `lm_labels` is changed to `labels` in a recent commit on master. If you are using master then use `labels` otherwise use `lm_labels` 

@sgugger This change is causing a lot of confusion. Would it be a good idea to keep master and release docs separate ?  <|||||>This has just been done. The [documentation](https://huggingface.co/transformers/) now shows the latest stable release (v2.11.0) and you have to opt-in to see the [master documentation](https://huggingface.co/transformers/master/).

I'll work on a version selector next.<|||||>Thanks @sgugger  !<|||||>I think we can close the issue as a result. Please reopen if any problem persists.