use format instead of f-strings

examples/run_lm_finetuning.py

@@ -57,7 +57,7 @@ class TextDataset(Dataset):
     def __init__(self, tokenizer, file_path='train', block_size=512):
         assert os.path.isfile(file_path)
         directory, filename = os.path.split(file_path)
-        cached_features_file = os.path.join(directory, 'cached_lm_{block_size}_{filename}')
+        cached_features_file = os.path.join(directory, 'cached_lm_{}_{}'.format(block_size, filename))
 
         if os.path.exists(cached_features_file):
             logger.info("Loading features from cached file %s", cached_features_file)