[trainer] `--load_best_model_at_end` silently turns of `--save_steps` settings
Splitting off from https://github.com/huggingface/transformers/pull/12477#discussion_r668326212

Currently `--load_best_model_at_end` silently turns off `--save_steps` settings when `--do_eval` is off (or `--evaluation_strategy` is  set to other than `"no"`, which otherwise automatically turns on `--do_eval`)

The proposal is to assert if: 

`--load_best_model_at_end` is set  and `--evaluation_strategy` is `"no"`

Reproducible test:

```
export BS=16; rm -r output_dir; PYTHONPATH=src USE_TF=0 CUDA_VISIBLE_DEVICES=0,1 python -m torch.distributed.launch --nproc_per_node=2 examples/pytorch/translation/run_translation.py --model_name_or_path t5-small --output_dir output_dir --adam_eps 1e-06  --do_train --label_smoothing 0.1 --learning_rate 3e-5 --logging_first_step --logging_steps 500 --max_source_length 128 --max_target_length 128 --val_max_target_length 128 --num_train_epochs 1 --overwrite_output_dir --per_device_train_batch_size $BS --predict_with_generate --sortish_sampler --source_lang en --target_lang ro --dataset_name wmt16 --dataset_config "ro-en" --source_prefix "translate English to Romanian: " --warmup_steps 50 --max_train_samples 50 --save_steps 1 
```
which saves checkpoints.

then adding `--load_best_model_at_end` stops saving those.

@sgugger.
Yes, as said in that comment, I think it's reasonable if we raise an error if `--load_best_model_at_end` is set and `--evaluation_strategy` is "no" since there is no "best model" to pick from in that case. I can do it later today if you want.<|||||>I'm still not 100% clear on how this feature's reliance on eval affects saving checkpoints, but if it solves the problem that's good enough for me.

Absolutely no rush on this one.

Thank you!<|||||>This issue has been automatically marked as stale because it has not had recent activity. If you think this still needs to be addressed please comment on this thread.

Please note that issues that do not follow the [contributing guidelines](https://github.com/huggingface/transformers/blob/master/CONTRIBUTING.md) are likely to be ignored.<|||||>Hasn't this one been resolved already?<|||||>Yes, this was fixed by #12786 in the end.