Can not import DataCollatorForLanguageModeling
# 🐛 Bug

## Information

Model I am using (ALBERT):

Language I am using the model on (Sanskrit, Hindi):

The problem arises when using:
* [x] the official example scripts: (give details below)
* [ ] my own modified scripts: (give details below)

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

## To reproduce

Steps to reproduce the behavior:

1. In Google Colab
2. ` !python /content/transformers/examples/run_language_modeling.py \
        --train_data_file /content/corpus/train/full.txt \
        --eval_data_file /content/corpus/valid/full_val.txt \
        --model_type albert-base-v2 \ `
3. This worked yesterdy, bbut the latest added `DataCollatorForLanguageModeling` can't be imported.

The error i am getting 

`2020-04-22 05:12:25.640328: I tensorflow/stream_executor/platform/default/dso_loader.cc:44] Successfully opened dynamic library libcudart.so.10.1
Traceback (most recent call last):
  File "/content/transformers/examples/run_language_modeling.py", line 29, in <module>
    from transformers import (
ImportError: cannot import name 'DataCollatorForLanguageModeling'`

So, I checked if it can be imported directly. 

`from transformers import DataCollatorForLanguageModeling`
 ERROR
`from transformers import DataCollatorForLanguageModeling
ImportError: cannot import name 'DataCollatorForLanguageModeling' `
## Expected behavior

<!-- 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: 2.8.0
- Platform: Colab
- Python version:  3.6.9
- PyTorch version (GPU?): 1.4.0
- Tensorflow version (GPU?): 2.2.0-rc3
- Using GPU in script?: Yes
- Using distributed or parallel set-up in script?: No

I was installing from pip and it isn't updated yet. Building from source solved this.<|||||>I am trying to run this from source and still I am getting the same error! 
Please have a look at this [here](https://github.com/huggingface/transformers/issues/3896#issue-604681806)<|||||>It's because the pip package hasn't been updated. The script to train is changed fundamentally. so you can try building from scratch using 
`git clone https://github.com/huggingface/transformers
cd transformers
pip install .` 
or 
You can use old script of `run_language_modeling.py` from previous commits. 