[DPR] Correct init (#13796)

* update

* add to docs and init

* make fix-copies

tests/test_modeling_dpr.py

@@ -14,6 +14,7 @@
 # limitations under the License.
 
 
+import tempfile
 import unittest
 
 from transformers import DPRConfig, is_torch_available
@@ -213,6 +214,19 @@ class DPRModelTest(ModelTesterMixin, unittest.TestCase):
         config_and_inputs = self.model_tester.prepare_config_and_inputs()
         self.model_tester.create_and_check_reader(*config_and_inputs)
 
+    def test_init_changed_config(self):
+        config = self.model_tester.prepare_config_and_inputs()[0]
+
+        model = DPRQuestionEncoder(config=config)
+        model.to(torch_device)
+        model.eval()
+
+        with tempfile.TemporaryDirectory() as tmp_dirname:
+            model.save_pretrained(tmp_dirname)
+            model = DPRQuestionEncoder.from_pretrained(tmp_dirname, projection_dim=512)
+
+        self.assertIsNotNone(model)
+
     @slow
     def test_model_from_pretrained(self):
         for model_name in DPR_CONTEXT_ENCODER_PRETRAINED_MODEL_ARCHIVE_LIST[:1]: