add dataset support for sampler, update batch

fastNLP/core/batch.py

@@ -56,8 +56,8 @@ class Batch(object):
             indices = self.idx_list[self.curidx:endidx]
 
             for field_name, field in self.dataset.get_fields():
-                batch = field.get(indices)
-                if not field.tensorable: #TODO 修改
+                batch = torch.from_numpy(field.get(indices))
+                if not field.need_tensor: #TODO 修改
                     pass
                 elif field.is_target:
                     batch_y[field_name] = batch

fastNLP/core/dataset.py

@@ -40,6 +40,13 @@ class DataSet(object):
                 assert name in self.field_arrays
                 self.field_arrays[name].append(field)
 
+    def get_fields(self):
+        return self.field_arrays
+
+    def __len__(self):
+        field = self.field_arrays.values()[0]
+        return len(field)
+
     def get_length(self):
         """Fetch lengths of all fields in all instances in a dataset.