[WIP] Trainer supporting evaluation on multiple datasets (#19158)

* support for multiple eval datasets

* support multiple datasets in seq2seq trainer

* add documentation

* update documentation

* make fixup

* revert option for multiple compute_metrics

* revert option for multiple compute_metrics

* revert added empty line

src/transformers/trainer.py

@@ -238,9 +238,10 @@ class Trainer:
             `torch.Generator` for the randomization that must be identical on all processes (and the Trainer will
             manually set the seed of this `generator` at each epoch) or have a `set_epoch()` method that internally
             sets the seed of the RNGs used.
-        eval_dataset (`torch.utils.data.Dataset`, *optional*):
+        eval_dataset (Union[`torch.utils.data.Dataset`, Dict[str, `torch.utils.data.Dataset`]), *optional*):
              The dataset to use for evaluation. If it is a [`~datasets.Dataset`], columns not accepted by the
-             `model.forward()` method are automatically removed.
+             `model.forward()` method are automatically removed. If it is a dictionary, it will evaluate on each
+             dataset prepending the dictionary key to the metric name.
         tokenizer ([`PreTrainedTokenizerBase`], *optional*):
             The tokenizer used to preprocess the data. If provided, will be used to automatically pad the inputs the
             maximum length when batching inputs, and it will be saved along the model to make it easier to rerun an
@@ -2040,6 +2041,14 @@ class Trainer:
 
         metrics = None
         if self.control.should_evaluate:
+            if isinstance(self.eval_dataset, dict):
+                for eval_dataset_name, eval_dataset in self.eval_dataset.items():
+                    metrics = self.evaluate(
+                        eval_dataset=eval_dataset,
+                        ignore_keys=ignore_keys_for_eval,
+                        metric_key_prefix=f"eval_{eval_dataset_name}",
+                    )
+            else:
                 metrics = self.evaluate(ignore_keys=ignore_keys_for_eval)
             self._report_to_hp_search(trial, self.state.global_step, metrics)