Respect sequence_len in config for `type: llama2_chat` (#926)

* Respect sequence_len in config for `type: llama2_chat`

It was hardcoded to `4096` I am not sure why? This updates it to pull from the config.

cc:

@winglian



* Update llama2_chat.py

* apply black formatting

* fix tokenizer

* update test data

* lint fixtures

src/axolotl/prompt_strategies/llama2_chat.py

@@ -81,8 +81,9 @@ class LLama2ChatTokenizingStrategy(PromptTokenizingStrategy):
 
     def __init__(self, *args, **kwargs):
         super().__init__(*args, **kwargs)
-        self.sequence_len = 4096
-        self.tokenizer.add_special_tokens({"pad_token": "<pad>"})
+        self.tokenizer.add_special_tokens(
+            {"pad_token": getattr(self.tokenizer, "pad_token", "<pad>")}
+        )
         # https://huggingface.co/meta-llama/Llama-2-7b-chat-hf/blob/main/added_tokens.json
 
     def tokenize_prompt(self, prompt):