I tried to compute the bert score using the IDF weighting but apparently it doesn't compute the IDF weights by itself.
The code is the following:

bertscore = evaluate.load("bertscore")
bertscore.compute(predictions=["This is a test", "Let's try if it works"], references=["Testing the solution", "Wondering if it is working"], lang="en", idf=True)

The error raised is AssertionError: IDF weights are not computed