Update usage doc regarding generate fn (#3504)

docs/source/usage.rst

@@ -420,7 +420,7 @@ to generate the tokens following the initial sequence in PyTorch, and creating a
     sequence = f"Hugging Face is based in DUMBO, New York City, and is"
 
     input = tokenizer.encode(sequence, return_tensors="pt")
-    generated = model.generate(input, max_length=50)
+    generated = model.generate(input, max_length=50, do_sample=True)
 
     resulting_string = tokenizer.decode(generated.tolist()[0])
     print(resulting_string)
@@ -432,14 +432,10 @@ to generate the tokens following the initial sequence in PyTorch, and creating a
     model = TFAutoModelWithLMHead.from_pretrained("gpt2")
 
     sequence = f"Hugging Face is based in DUMBO, New York City, and is"
-    generated = tokenizer.encode(sequence)
-
-    for i in range(50):
-        predictions = model(tf.constant([generated]))[0]
-        token = tf.argmax(predictions[0], axis=1)[-1].numpy()
-        generated += [token]
+    input = tokenizer.encode(sequence, return_tensors="tf")
+    generated = model.generate(input, max_length=50, do_sample=True)
 
-    resulting_string = tokenizer.decode(generated)
+    resulting_string = tokenizer.decode(generated.tolist()[0])
     print(resulting_string)