adding key pair dataset (#19765)

docs/source/en/main_classes/pipelines.mdx

@@ -91,7 +91,7 @@ pipe = pipeline("automatic-speech-recognition", model="facebook/wav2vec2-base-96
 dataset = datasets.load_dataset("superb", name="asr", split="test")
 
 # KeyDataset (only *pt*) will simply return the item in the dict returned by the dataset item
-# as we're not interested in the *target* part of the dataset.
+# as we're not interested in the *target* part of the dataset. For sentence pair use KeyPairDataset
 for out in tqdm(pipe(KeyDataset(dataset, "file"))):
     print(out)
     # {"text": "NUMBER TEN FRESH NELLY IS WAITING ON YOU GOOD NIGHT HUSBAND"}

src/transformers/pipelines/pt_utils.py

@@ -293,3 +293,16 @@ class KeyDataset(Dataset):
 
     def __getitem__(self, i):
         return self.dataset[i][self.key]
+
+
+class KeyPairDataset(Dataset):
+    def __init__(self, dataset: Dataset, key1: str, key2: str):
+        self.dataset = dataset
+        self.key1 = key1
+        self.key2 = key2
+
+    def __len__(self):
+        return len(self.dataset)
+
+    def __getitem__(self, i):
+        return {"text": self.dataset[i][self.key1], "text_pair": self.dataset[i][self.key2]}