Add new benchmark: Portuguese bench (#2156)

* Add portuguese_bench

* Add flores_pt group

* Update _flores_common_yaml

* Run linters and update flores and readme

lm_eval/tasks/portuguese_bench/flores_pt/flores_pt-gl.yaml

+# File generated by `create-yamls.py`
+include: _flores_common_yaml
+task: flores_pt-gl
+doc_to_text: 'Portuguese sentence: {{sentence_por_Latn}}
+
+  Galician sentence:'
+doc_to_target: '{{sentence_glg_Latn}}'

lm_eval/tasks/portuguese_bench/flores_pt/flores_pt-it.yaml

+# File generated by `create-yamls.py`
+include: _flores_common_yaml
+task: flores_pt-it
+doc_to_text: 'Portuguese sentence: {{sentence_por_Latn}}
+
+  Italian sentence:'
+doc_to_target: '{{sentence_ita_Latn}}'

lm_eval/tasks/portuguese_bench/flores_pt/flores_pt.yaml

+group: flores_pt
+task:
+  - flores_es-pt
+  - flores_pt-es
+  - flores_en-pt
+  - flores_pt-en
+  - flores_eu-pt
+  - flores_pt-eu
+  - flores_pt-it
+  - flores_it-pt
+  - flores_pt-fr
+  - flores_fr-pt
+  - flores_pt-ca
+  - flores_ca-pt
+  - flores_pt-gl
+  - flores_gl-pt
+  - flores_pt-de
+  - flores_de-pt
+aggregate_metric_list:
+  - metric: bleu
+    aggregation: mean
+    weight_by_size: false
+metadata:
+  version: 1.0

lm_eval/tasks/portuguese_bench/portuguese_bench.yaml

+group: portuguese_bench
+task:
+  - belebele_por_Latn
+  - flores_pt
+  - assin_paraphrase
+  - assin_entailment
+metadata:
+  version: 1.0