feat(videomme_v2): add reasoning mode variant

- Chain-of-thought prompt requiring Final Answer: <letter> format
- max_new_tokens=4096 for reasoning space
- Task: videomme_v2_reasoning

Author: mwxely

lmms_eval/tasks/videomme_v2/videomme_v2_reasoning.yaml

@@ -0,0 +1,9 @@
+task: videomme_v2_reasoning
+include: _default_template_yaml
+doc_to_text: !function utils.videomme_v2_doc_to_text_reasoning
+generation_kwargs:
+  max_new_tokens: 4096
+  temperature: 0
+  top_p: 1.0
+  num_beams: 1
+  do_sample: false