Avoid looping when data exhausted
# What does this PR do?

This fix avoids running into a virtually infinite loop when using a finite iterable dataset.

When using an iterable dataset `num_epochs` is set to sys.maxsize to make sure all data is consumed (see https://github.com/huggingface/transformers/pull/12561)
Likewise I'd like to set `max_steps` large enough to consume all data but still stop when the data is exhausted. In case we don't know how many samples there will be and the iterator stops we might run into a virtually infinite loop (iterating the int range until `sys.maxsize`).

See this code snipped to reproduce the behavior:

```python
from torch.utils.data import IterableDataset

from transformers import BertForMaskedLM, BertConfig, TrainingArguments, Trainer

model = BertForMaskedLM(BertConfig())


class FiniteIterableDataset(IterableDataset):
    def __init__(self, num_samples: int):
        self.current_sample = 0
        self.num_samples = num_samples

    def __iter__(self):
        while self.current_sample < self.num_samples:
            yield {"input_ids": [0, 0, 0, self.current_sample], "labels": [0, 0, 0, 1]}
            self.current_sample += 1


batch_size = 1
gradient_accumulation_steps = 1
num_samples = 10

available_steps = num_samples // (batch_size * gradient_accumulation_steps)

data = FiniteIterableDataset(num_samples)
train_args = TrainingArguments(
    "tmp_dir",
    max_steps=available_steps,
    per_device_train_batch_size=batch_size,
    gradient_accumulation_steps=gradient_accumulation_steps,
)
trainer = Trainer(model, train_dataset=data, args=train_args)
trainer.train()  # works

data = FiniteIterableDataset(num_samples)
train_args = TrainingArguments(
    "tmp_dir",
    max_steps=available_steps + 1,  # set a higher number than actually available
    per_device_train_batch_size=batch_size,
    gradient_accumulation_steps=gradient_accumulation_steps,
)
trainer = Trainer(model, train_dataset=data, args=train_args)
trainer.train()  # "hangs" at 91% after 10 steps iterating through epochs like wild (until sys.maxsize)
```

With this fix it is checked whether `epoch_iterator` did not produce any samples and accordingly set `control.should_training_stop` to `True`.
I don't know if changing the flow control this way is approved of as it's always changed through callback handlers, I'm happy for suggestions how to properly do this.

I tried coming up with a test case checking the logs for when training was stopped in this case. Other options would be to measure the time training takes and time out after a while but that wouldn't be a nice test as run time may be affected by other circumstances.


## Before submitting
- [ ] This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
- [x] Did you read the [contributor guideline](https://github.com/huggingface/transformers/blob/master/CONTRIBUTING.md#start-contributing-pull-requests), Pull Request section?
- [ ] Was this discussed/approved via a Github issue or the [forum](https://discuss.huggingface.co/)? Please add a link to it if that's the case.
- [x] Did you make sure to update the documentation with your changes? Here are the [documentation guidelines] https://github.com/huggingface/transformers/tree/master/docs), and [here are tips on formatting docstrings](https://github.com/huggingface/transformers/tree/master/docs#writing-source-documentation).
- [x] Did you write any new necessary tests?


## Who can review?

- trainer: @sgugger


Thanks again for fixing this! :-)