[trainer] sanity checks for `save_steps=0|None` and `logging_steps=0`
This PR deals with combinations of training args that lead to:


```
# --do_eval --evaluation_strategy=steps --logging_steps 0 (no --eval_args)
Traceback (most recent call last):
  File "examples/pytorch/translation/run_translation.py", line 617, in <module>
    main()
  File "examples/pytorch/translation/run_translation.py", line 537, in main
    train_result = trainer.train(resume_from_checkpoint=checkpoint)
  File "/mnt/nvme1/code/huggingface/transformers-master/src/transformers/trainer.py", line 1340, in train
    self.control = self.callback_handler.on_step_end(args, self.state, self.control)
  File "/mnt/nvme1/code/huggingface/transformers-master/src/transformers/trainer_callback.py", line 359, in on_step_end
    return self.call_event("on_step_end", args, state, control)
  File "/mnt/nvme1/code/huggingface/transformers-master/src/transformers/trainer_callback.py", line 378, in call_event
    result = getattr(callback, event)(
  File "/mnt/nvme1/code/huggingface/transformers-master/src/transformers/trainer_callback.py", line 414, in on_step_end
    if args.evaluation_strategy == IntervalStrategy.STEPS and state.global_step % args.eval_steps == 0:
ZeroDivisionError: integer division or modulo by zero
```

and:

```
# --do_eval --evaluation_strategy=steps --logging_steps 0 --eval_args 5
Traceback (most recent call last):
  File "examples/pytorch/translation/run_translation.py", line 617, in <module>
    main()
  File "examples/pytorch/translation/run_translation.py", line 537, in main
    train_result = trainer.train(resume_from_checkpoint=checkpoint)
  File "/mnt/nvme1/code/huggingface/transformers-master/src/transformers/trainer.py", line 1343, in train
    self.control = self.callback_handler.on_step_end(args, self.state, self.control)
  File "/mnt/nvme1/code/huggingface/transformers-master/src/transformers/trainer_callback.py", line 359, in on_step_end
    return self.call_event("on_step_end", args, state, control)
  File "/mnt/nvme1/code/huggingface/transformers-master/src/transformers/trainer_callback.py", line 378, in call_event
    result = getattr(callback, event)(
  File "/mnt/nvme1/code/huggingface/transformers-master/src/transformers/trainer_callback.py", line 406, in on_step_end
    if args.logging_strategy == IntervalStrategy.STEPS and state.global_step % args.logging_steps == 0:
ZeroDivisionError: integer division or modulo by zero
```
(this last one doesn't happen with master, but I reworked things to do the same enforcement in init as for `eval_steps`)

@sgugger 

All good to merge, thanks again!