fix t5 training docstring (#7911)

docs/source/model_doc/t5.rst

@@ -62,10 +62,10 @@ token. T5 can be trained / fine-tuned both in a supervised and unsupervised fash
 
 .. code-block::
 
-  input_ids = tokenizer.encode('The <extra_id_0> walks in <extra_id_1> park', return_tensors='pt')
-  labels = tokenizer.encode('<extra_id_0> cute dog <extra_id_1> the <extra_id_2> </s>', return_tensors='pt')
+  input_ids = tokenizer('The <extra_id_0> walks in <extra_id_1> park', return_tensors='pt').input_ids
+  labels = tokenizer('<extra_id_0> cute dog <extra_id_1> the <extra_id_2>', return_tensors='pt').input_ids
   # the forward function automatically creates the correct decoder_input_ids
-  model(input_ids=input_ids, labels=labels)
+  loss = model(input_ids=input_ids, labels=labels, return_dict=True).loss
 
 - Supervised training
 
@@ -75,10 +75,10 @@ token. T5 can be trained / fine-tuned both in a supervised and unsupervised fash
   
 .. code-block::
 
-  input_ids = tokenizer.encode('translate English to German: The house is wonderful. </s>', return_tensors='pt')
-  labels = tokenizer.encode('Das Haus ist wunderbar. </s>', return_tensors='pt')
+  input_ids = tokenizer('translate English to German: The house is wonderful.', return_tensors='pt').input_ids
+  labels = tokenizer('Das Haus ist wunderbar.', return_tensors='pt').input_ids
   # the forward function automatically creates the correct decoder_input_ids
-  model(input_ids=input_ids, labels=labels)
+  loss = model(input_ids=input_ids, labels=labels, return_dict=True).loss
 
 
 T5Config