Run inference with decoder-only models with the text-generation pipeline:
thon

from transformers import pipeline
import torch
torch.manual_seed(0) # doctest: +IGNORE_RESULT
generator = pipeline('text-generation', model = 'openai-community/gpt2')
prompt = "Hello, I'm a language model"
generator(prompt, max_length = 30)
[{'generated_text': "Hello, I'm a language model expert, so I'm a big believer in the concept that I know very well and then I try to look into"}]

To run inference with an encoder-decoder, use the text2text-generation pipeline:
thon

text2text_generator = pipeline("text2text-generation", model = 'google/flan-t5-base')
prompt = "Translate from English to French: I'm very happy to see you"
text2text_generator(prompt)
[{'generated_text': 'Je suis très heureuse de vous rencontrer.'}]