Error in PyTorch Trainer when used with TPU
## 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: 3.0.2
- Platform: Linux-4.19.112+-x86_64-with-Ubuntu-18.04-bionic
- Python version: 3.6.9
- PyTorch version (GPU?): 1.5.0a0+d6149a7 (False)
- Tensorflow version (GPU?): 2.3.0 (False)
- Using GPU in script?: No
- Using distributed or parallel set-up in script?: No

### Who can help
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 albert, bert, GPT2, XLM: @LysandreJik 
 tokenizers: @mfuntowicz
 Trainer: @sgugger
 Speed and Memory Benchmarks: @patrickvonplaten
 Model Cards: @julien-c
 Translation: @sshleifer
 Summarization: @sshleifer
 TextGeneration: @TevenLeScao 
 examples/distillation: @VictorSanh
 nlp datasets: [different repo](https://github.com/huggingface/nlp)
 rust tokenizers: [different repo](https://github.com/huggingface/tokenizers)
 Text Generation: @TevenLeScao
 blenderbot: @mariamabarham
 Bart: @sshleifer
 Marian: @sshleifer
 T5: @patrickvonplaten
 Longformer/Reformer: @patrickvonplaten
 TransfoXL/XLNet: @TevenLeScao 
 examples/seq2seq: @sshleifer
 tensorflow: @jplu 
documentation: @sgugger
 --> @sgugger

## Information

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

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

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

The following error arises when using the `run_squad_trainer.py` script with TPU:
```python
Epoch:   0% 0/2 [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/question-answering/run_squad_trainer.py", line 156, in _mp_fn
    main()
  File "/content/transformers/examples/question-answering/run_squad_trainer.py", line 145, in main
    model_path=model_args.model_name_or_path if os.path.isdir(model_args.model_name_or_path) else None
  File "/usr/local/lib/python3.6/dist-packages/transformers/trainer.py", line 584, in train
    self.epoch = epoch + (step + 1) / len(epoch_iterator)
TypeError: 'NoneType' object cannot be interpreted as an integer
```

## To reproduce

Steps to reproduce the behavior:

1. install transformers from the master branch
2. install pytorch-xla using the following command:
```shell
VERSION = "20200325"
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
```
3. run the training script (I'm using 1 tpu core merely to simplify the logs. The error is the same (for each core) when using 8 cores):
```shell
cd transformers/examples/

python ./xla_spawn.py --num_cores 1 \
question-answering/run_squad_trainer.py \
    --model_name_or_path bert-base-multilingual-cased \
    --model_type bert \
    --data_dir $DATA_DIR \
    --do_train \
    --per_device_train_batch_size 64 \
    --learning_rate 3e-5 \
    --num_train_epochs 2.0 \
    --max_seq_length 384 \
    --doc_stride 128 \
    --output_dir $OUT_DIR \
    --overwrite_output_dir
```

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## Expected behavior
The script runs and trains the model
<!-- A clear and concise description of what you would expect to happen. -->

I am receiving the same error. Even without using TPU. 

python run_glue.py   --model_name_or_path bert-base-cased   --task_name MRPC   --do_train   --do_eval   --data_dir $GLUE_DIR/MRPC/   --max_seq_length 128   --per_device_train_batch_size    --learning_rate 2e-5   --num_train_epochs 3.0   --output_dir /tmp/mrpc_output/ 
<|||||>Try with the following:

```bash
!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 "nightly"

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

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

!python transformers/examples/xla_spawn.py --num_cores 1 \
    question-answering/run_squad_trainer.py \
    --model_name_or_path bert-base-multilingual-cased \
    --model_type bert \
    --data_dir $DATA_DIR \
    --do_train \
    --per_device_train_batch_size 64 \
    --learning_rate 3e-5 \
    --num_train_epochs 2.0 \
    --max_seq_length 384 \
    --doc_stride 128 \
    --output_dir $OUT_DIR \
    --overwrite_output_dir
```

To run it with all `8 TPU` cores, you most likely need the `35GB RAM` runtime from Google Colab. You can find it in this [notebook](https://colab.research.google.com/github/patil-suraj/exploring-T5/blob/master/T5_on_TPU.ipynb#scrollTo=QLGiFCDqvuil).<|||||>Thanks @AliOsm, it works!