[FlaxT5 Example] fix flax t5 example pretraining (#15835)

examples/flax/language-modeling/run_t5_mlm_flax.py

@@ -368,7 +368,9 @@ class FlaxDataCollatorForT5MLM:
         batch_size = input_ids.shape[0]
 
         input_ids_full = np.where(sentinel_ids != 0, sentinel_ids, input_ids)
-        input_ids = input_ids_full[input_ids_full > 0].reshape((batch_size, -1))
+        # input_ids tokens and sentinel tokens are >= 0, tokens < 0 are
+        # masked tokens coming after sentinel tokens and should be removed
+        input_ids = input_ids_full[input_ids_full >= 0].reshape((batch_size, -1))
         input_ids = np.concatenate(
             [input_ids, np.full((batch_size, 1), self.tokenizer.eos_token_id, dtype=np.int32)], axis=-1
         )