add unit tests for cum seq lens, add ability to build cu_seq_lens from positional ids, fix prompt test

tests/test_prompt_tokenizers.py

@@ -134,9 +134,15 @@ class InstructionWSystemPromptTokenizingStrategyTest(unittest.TestCase):
             "output": "Hi! How can I help?",
         }
         example = strat.tokenize_prompt(sample)
-        assert example["input_ids"][0:4] == [1, 835, 2184, 29901]  # "<s>### System:"
-        assert example["input_ids"][5:7] == [1509, 20118]  # "use cot"
-        assert example["input_ids"][9] == 11889  # USER
+        assert example["input_ids"][0:5] == [
+            1,
+            28962,
+            1254,
+            12665,
+            29901,
+        ]  # "<s>SYSTEM:"
+        assert example["input_ids"][5:7] == [671, 20118]  # " use cot"
+        assert example["input_ids"][8] == 11889  # USER
 
 
 class Llama2ChatTokenizationTest(unittest.TestCase):