Migrate metrics used in flax examples to Evaluate (#18348)

Currently, tensorflow examples use the `load_metric` function from
Datasets library, commit migrates function call to `load` function
from Evaluate library.

examples/flax/summarization/run_summarization_flax.py

@@ -33,9 +33,10 @@ from typing import Callable, Optional
 import datasets
 import nltk  # Here to have a nice missing dependency error message early on
 import numpy as np
-from datasets import Dataset, load_dataset, load_metric
+from datasets import Dataset, load_dataset
 from tqdm import tqdm
 
+import evaluate
 import jax
 import jax.numpy as jnp
 import optax
@@ -656,7 +657,7 @@ def main():
         )
 
     # Metric
-    metric = load_metric("rouge")
+    metric = evaluate.load("rouge")
 
     def postprocess_text(preds, labels):
         preds = [pred.strip() for pred in preds]