Cast logits to fp32 at the end of TF_T5 (#12332)

This change enables tf.keras.mixed_precision with bf16

src/transformers/models/t5/modeling_tf_t5.py

@@ -1407,6 +1407,8 @@ class TFT5ForConditionalGeneration(TFT5PreTrainedModel, TFCausalLanguageModeling
         else:
             logits = self.lm_head(sequence_output)
 
+        logits = tf.cast(logits, tf.float32)
+
         loss = None if inputs["labels"] is None else self.compute_loss(inputs["labels"], logits)
 
         if not inputs["return_dict"]: