[bug] use_gather_object is not respected after the first eval in trainer

[ducha-aiki]

Hi,

Consider this as a mix of bug report and PR. Unfortunately, it would take too long for me to cook the toy example to reproduce, and the issue is quite simple and obvious enough.

So, there is a use_gather_object argument in TrainingArguments, allowing to use non-tensors in eval, or tensors with different shapes.
It is handled here:

https://github.com/huggingface/transformers/blob/main/src/transformers/trainer.py#L5103

        # create accelerator object
        self.accelerator = Accelerator(**args)
        # some Trainer classes need to use `gather` instead of `gather_for_metrics`, thus we store a flag
        self.gather_function = self.accelerator.gather_for_metrics

        if "use_gather_object" in inspect.signature(self.gather_function).parameters.keys():
            self.gather_function = functools.partial(
                self.gather_function, use_gather_object=self.args.eval_use_gather_object
            )

However, I have noticed, that after the first eval, the 2nd eval is crashing, while trying to concat batches with different shapes, as if the flag eval_use_gather_object stopped working. Indeed it does, because here in evaluation_loop, the self.gather_function is reset, and the .eval_use_gather_object is not used
https://github.com/huggingface/transformers/blob/main/src/transformers/trainer.py#L4359

        # After all calls to `.gather_function`, reset to `gather_for_metrics`:
        self.gather_function = self.accelerator.gather_for_metrics

Suggested fix (I am using it myself): add the same line to use the flag.
https://github.com/huggingface/transformers/blob/main/src/transformers/trainer.py#L4359

        if "use_gather_object" in inspect.signature(self.gather_function).parameters.keys():
            self.gather_function = functools.partial(
                self.gather_function, use_gather_object=self.args.eval_use_gather_object
            )

[SunMarc]

Hi @ducha-aiki, thanks for the report ! I just reviewed your PR