Change assertion to warning when passing past_key_value to T5 encoder (#16153)

* Change assertion to warning when passing past_key_value to T5 encoder

* lint

src/transformers/models/t5/modeling_t5.py

@@ -648,7 +648,8 @@ class T5Block(nn.Module):
     ):
 
         if past_key_value is not None:
-            assert self.is_decoder, "Only decoder can use `past_key_values`"
+            if not self.is_decoder:
+                logger.warning("`past_key_values` is passed to the encoder. Please make sure this is intended.")
             expected_num_past_key_values = 2 if encoder_hidden_states is None else 4
 
             if len(past_key_value) != expected_num_past_key_values: