Run Language Modeling on Colab TPU cores terminates
# 🐛 Bug

## Information

Model I am using (Bert, XLNet ...): GPT2

Language I am using the model on (English, Chinese ...): English (wikitext-2)

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)

I'm trying to test `run_language_modeling.py` on GPT2 using all 8 TPU cores.

Running on 1 core gives the following error:
```bash
Epoch:   0% 0/3 [00:00<?, ?it/s]
Iteration: 0it [00:00, ?it/s]Exception in device=TPU:0: 'NoneType' object cannot be interpreted as an integer
Traceback (most recent call last):
  File "/usr/local/lib/python3.6/dist-packages/torch_xla/distributed/xla_multiprocessing.py", line 119, in _start_fn
    fn(gindex, *args)
  File "/content/transformers/examples/language-modeling/run_language_modeling.py", line 292, in _mp_fn
    main()
  File "/content/transformers/examples/language-modeling/run_language_modeling.py", line 260, in main
    trainer.train(model_path=model_path)
  File "/usr/local/lib/python3.6/dist-packages/transformers/trainer.py", line 519, in train
    self.epoch = epoch + (step + 1) / len(epoch_iterator)
TypeError: 'NoneType' object cannot be interpreted as an integer
```

While running using all 8 cores gives this one:
```bash
/usr/lib/python3.6/multiprocessing/semaphore_tracker.py:143: UserWarning: semaphore_tracker: There appear to be 1 leaked semaphores to clean up at shutdown
  len(cache))
Traceback (most recent call last):
  File "transformers/examples/xla_spawn.py", line 72, in <module>
    main()
  File "transformers/examples/xla_spawn.py", line 68, in main
    xmp.spawn(mod._mp_fn, args=(), nprocs=args.num_cores)
  File "/usr/local/lib/python3.6/dist-packages/torch_xla/distributed/xla_multiprocessing.py", line 182, in spawn
    start_method=start_method)
  File "/usr/local/lib/python3.6/dist-packages/torch/multiprocessing/spawn.py", line 158, in start_processes
    while not context.join():
  File "/usr/local/lib/python3.6/dist-packages/torch/multiprocessing/spawn.py", line 108, in join
    (error_index, name)
Exception: process 0 terminated with signal SIGKILL
```

I'm running this on a Colab TPU Notebook.

## To reproduce

Steps to reproduce the behavior:

```python
VERSION = "20200325"  #@param ["1.5" , "20200325", "nightly"]
!curl https://raw.githubusercontent.com/pytorch/xla/master/contrib/scripts/env-setup.py -o pytorch-xla-env-setup.py
!python pytorch-xla-env-setup.py --version $VERSION
import torch_xla
import torch_xla.core.xla_model as xm

!pip install git+https://github.com/huggingface/transformers.git

!git clone https://github.com/huggingface/transformers.git

!curl https://s3.amazonaws.com/research.metamind.io/wikitext/wikitext-2-v1.zip --output wikitext-2-v1.zip
!unzip wikitext-2-v1.zip
!rm wikitext-2-v1.zip

!python transformers/examples/xla_spawn.py --num_cores 1 \
	transformers/examples/language-modeling/run_language_modeling.py \
    --output_dir=output \
    --model_type=gpt2 \
    --model_name_or_path=gpt2 \
    --do_train \
    --train_data_file=wikitext-2/wiki.train.tokens \
    --do_eval \
    --eval_data_file=wikitext-2/wiki.test.tokens \
    --per_device_train_batch_size 1
```

## Expected behavior

Finetuning the model and saves it.

## Environment info

- `transformers` version: 3.0.2
- Platform: Linux-4.19.104+-x86_64-with-Ubuntu-18.04-bionic
- Python version: 3.6.9
- PyTorch version (GPU?): 1.5.0a0+d6149a7 (False)
- Tensorflow version (GPU?): 2.2.0 (False)
- Using GPU in script?: No
- Using distributed or parallel set-up in script?: Yes and No

For the single core problem, I think it is related to how the trainer prepares the `epoch_loader` object [here](https://github.com/huggingface/transformers/blob/master/src/transformers/trainer.py#L480-L484).<|||||>Update: for the single core problem, removing ` / len(epoch_iterator)` part from this [line](https://github.com/huggingface/transformers/blob/master/src/transformers/trainer.py#L519) solves the problem, so I suggest to precompute the value using `len(train_loader)` before this [if statement](https://github.com/huggingface/transformers/blob/master/src/transformers/trainer.py#L480) and use it later. The multicores problem is still exists, could it relate to RAM limits in Google Colab?<|||||>This indeed seems to be a problem. I encountered the same issue.
The hack suggested by @AliOsm seems to work for now<|||||>@julien-c could you please take a look?<|||||>The memory was the problem! In the beginning of the notebook, run the following cell to get the 35GB RAMs runtime instead of the 12GB one:

```python
import torch
torch.tensor([10.]*10000000000)
```

Then, use this snippet of code to finetune GPT-2 on wikitext-2:

```bash
VERSION = "nightly"  #@param ["1.5" , "20200325", "nightly"]
!curl https://raw.githubusercontent.com/pytorch/xla/master/contrib/scripts/env-setup.py -o pytorch-xla-env-setup.py
!python pytorch-xla-env-setup.py --version $VERSION

!pip install git+https://github.com/huggingface/transformers.git

!git clone https://github.com/huggingface/transformers.git

!curl https://s3.amazonaws.com/research.metamind.io/wikitext/wikitext-2-v1.zip --output wikitext-2-v1.zip
!unzip wikitext-2-v1.zip
!rm wikitext-2-v1.zip

!python transformers/examples/xla_spawn.py --num_cores 8 \
	transformers/examples/language-modeling/run_language_modeling.py \
    --output_dir=output \
    --model_type=gpt2 \
    --model_name_or_path=gpt2 \
    --do_train \
    --train_data_file=wikitext-2/wiki.train.tokens \
    --do_eval \
    --eval_data_file=wikitext-2/wiki.test.tokens \
    --per_device_train_batch_size 2 \
    --overwrite_output_dir
```

It will be helpful to put this in the documentation :3