Enable StagingEngine in test_engine_paged_cache_model.py

[masahi]

Expected output

$ python serve/tests/test_engine_paged_cache_model.py  --local-id vicuna-v1-7b-q0f16 --max-num-batched-tokens 2560 --max-input-len 256   --use-staging-engine
Prompt = 'Hello, my name is', generated text = 'Dr. David C. Preston, and I am a chiropractor serving the residents of the'
Prompt = 'The president of the United States is', generated text = 'a man who has been accused of sexual assault by more than a dozen women, and who has b'
Prompt = 'The capital of France is', generated text = 'Paris.
The currency of France is the Euro.
The official language of France is French.'
Prompt = 'The future of AI is', generated text = 'bright, and it’s up to us to ensure that it benefits everyone. As we continue to'

Currently using SyncEngine (without --use-staging-engine) is broken:

Prompt = 'Hello, my name is', generated text = ''
Prompt = 'The president of the United States is', generated text = ''
Prompt = 'The capital of France is', generated text = ''
Prompt = 'The future of AI is', generated text = 'DraParisbright. man., David who
 and C hasThe it. been currency’ Pr accused ofseston of France up, sexual is to and assault the us I by Euro to am more. ensure a than
 that chi aThe itrop dozen official benefitsractor women language everyone serving, of. the and France As residents who is we of has French continue the b. to'