TrainingArguments._n_gpus serialization
I'm not sure, but I think that `TrainingArguments._n_gpu` attribute is not supposed to be serialized in `TrainingArguments.to_dict()` .
Or even basically all the attributes that are defined as `dataclass.field(init=False...)` should be filtered out while doing `to_dict()`.

What do you think?

@sgugger 
Indeed, this argument shouldn't be serialized.<|||||>I'm on it