Add __len__ to squad dataset (#4989)

* update lrscheduler

* minor fix

* add pre-commit

* minor fix

* Add __len__ to squad dataset

* minor fix

PaddleNLP/paddlenlp/datasets/squad.py

@@ -445,6 +445,9 @@ class SQuAD(Dataset):
 
         self.examples = examples
 
+    def __len__(self):
+        return len(self.data)
+
     def __getitem__(self, idx):
         feature = self.data[idx]