Truncate MCQ inputs from the start and not end (#1256)

* Truncate MCQ inputs from the start and not end

In MCQ setting, the default truncation shouldn't be from the end. It has a risk of removing the question altogether. It's a better default to truncate from start so that if anything part of the context gets truncated but not the question.

Co-authored-by: jeswan <57466294+jeswan@users.noreply.github.com>

jiant/tasks/lib/templates/multiple_choice.py

@@ -65,6 +65,7 @@ def featurize(self, tokenizer, feat_spec):
             prompt, choice = truncate_sequences(
                 tokens_ls=[self.prompt, choice],
                 max_length=feat_spec.max_seq_length - special_tokens_count,
+                truncate_end=False,
             )
             unpadded_inputs = add_cls_token(
                 unpadded_tokens=(