T5ForConditionalGeneration: enabling using past_key_values and labels in training (#13805)

* enabling using past_key_values together with labels when training in T5ForConditionalGeneration

* test

* Enable past_key_values in T5ForconditionalGeneration while training.

* delete comments

src/transformers/models/t5/modeling_t5.py

@@ -1593,15 +1593,6 @@ class T5ForConditionalGeneration(T5PreTrainedModel):
             # get decoder inputs from shifting lm labels to the right
             decoder_input_ids = self._shift_right(labels)
 
-        # If decoding with past key value states, only the last tokens
-        # should be given as an input
-        if past_key_values is not None:
-            assert labels is None, "Decoder should not use cached key value states when training."
-            if decoder_input_ids is not None:
-                decoder_input_ids = decoder_input_ids[:, -1:]
-            if decoder_inputs_embeds is not None:
-                decoder_inputs_embeds = decoder_inputs_embeds[:, -1:]
-
         # Set device for model parallelism
         if self.model_parallel:
             torch.cuda.set_device(self.decoder.first_device)