MLM training fails with no validation file(same as #12406 for pytorch now)
## Environment info

- `transformers` version: 4.9.0.dev0
- Platform: Linux-5.4.104+-x86_64-with-Ubuntu-18.04-bionic
- Python version: 3.7.10
- PyTorch version (GPU?): 1.9.0+cu102 (True)
- Tensorflow version (GPU?): 2.5.0 (True)
- Flax version (CPU?/GPU?/TPU?): not installed (NA)
- Jax version: not installed
- JaxLib version: not installed
- Using GPU in script?: Yes
- Using distributed or parallel set-up in script?: Yes



## Information

Model I am using (Bert, XLNet ...): distilbert-base-cased

The problem arises when using: the official example scripts: (give details below)

The tasks I am working on is: MLM finetuning

## To reproduce

Steps to reproduce the behavior:

1. Just run the tensorflow  examples
2. python3 ./transformers/examples/pytorch/language-modeling/run_mlm.py\
 --model_name_or_path distilbert-base-cased \
 --output_dir ./g \
 --train_file "customdata.txt" \
3. The model fails with error message that no validation file is there.


## Expected behavior
it should use the validation split percentage parameter to divide the training set into training and eval samples.
@sgugger @patil-suraj ..
Checks done..