[finetune_trainer] --evaluate_during_training is no more
In `examples/seq2seq/builtin_trainer/` all scripts reference `--evaluate_during_training ` but it doesn't exist in pt trainer, but does exist in tf trainer:
```
grep -Ir evaluate_during
builtin_trainer/finetune.sh:    --do_train --do_eval --do_predict --evaluate_during_training \
builtin_trainer/train_distil_marian_enro.sh:    --do_train --do_eval --do_predict --evaluate_during_training\
builtin_trainer/finetune_tpu.sh:    --do_train --do_eval --evaluate_during_training \
builtin_trainer/train_distilbart_cnn.sh:    --do_train --do_eval --do_predict --evaluate_during_training \
builtin_trainer/train_distil_marian_enro_tpu.sh:    --do_train --do_eval --evaluate_during_training \
builtin_trainer/train_mbart_cc25_enro.sh:    --do_train --do_eval --do_predict --evaluate_during_training \
```

```
Traceback (most recent call last):
  File "finetune_trainer.py", line 310, in <module>
    main()
  File "finetune_trainer.py", line 118, in main
    model_args, data_args, training_args = parser.parse_args_into_dataclasses()
  File "/home/stas/anaconda3/envs/main-38/lib/python3.8/site-packages/transformers/hf_argparser.py", line 144, in parse_args_into_dataclasses
    raise ValueError(f"Some specified arguments are not used by the HfArgumentParser: {remaining_args}")
ValueError: Some specified arguments are not used by the HfArgumentParser: ['--evaluate_during_training']
```
Is this meant to be replaced by: `--evaluation_strategy` - this is the closest I found in `training_args.py`

If so which one? `steps` or `epoch`?

Also the help output is borked:

```
$ python finetune_trainer.py -h
...
 [--evaluation_strategy {EvaluationStrategy.NO,EvaluationStrategy.STEPS,EvaluationStrategy.EPOCH}]
```
probably this is not what what's intended, but 
```
 [--evaluation_strategy {no, steps, epochs}
```

But perhaps it's a bigger issue - I see `trainer.args.evaluate_during_training`:
```
src/transformers/integrations.py:        ) and (not trainer.args.do_eval or not trainer.args.evaluate_during_training):
```
and also `--evaluate_during_training` in many other files under `examples/`.

Thank you.

@sgugger, @patrickvonplaten 

Found the source of breakage: https://github.com/huggingface/transformers/pull/8604 - I guess that PR needs more work