Create method to save instances to cache file. (#3131)

Allows other dataset readers to override, e.g. to use pickle instead of
jsonpickle.

allennlp/data/dataset_readers/dataset_reader.py

@@ -139,9 +139,8 @@ def read(self, file_path: str) -> Iterable[Instance]:
             # And finally we write to the cache if we need to.
             if cache_file and not os.path.exists(cache_file):
                 logger.info(f"Caching instances to {cache_file}")
-                with open(cache_file, 'w') as cache:
-                    for instance in Tqdm.tqdm(instances):
-                        cache.write(self.serialize_instance(instance) + '\n')
+                self._instances_to_cache_file(cache_file, instances)
+
             return instances
 
     def _get_cache_location_for_file_path(self, file_path: str) -> str:
@@ -161,6 +160,11 @@ def _instances_from_cache_file(self, cache_filename: str) -> Iterable[Instance]:
             for line in cache_file:
                 yield self.deserialize_instance(line.strip())
 
+    def _instances_to_cache_file(self, cache_filename, instances) -> None:
+        with open(cache_filename, 'w') as cache:
+            for instance in Tqdm.tqdm(instances):
+                cache.write(self.serialize_instance(instance) + '\n')
+
     def text_to_instance(self, *inputs) -> Instance:
         """
         Does whatever tokenization or processing is necessary to go from textual input to an