[Fix] fix import error in quick run demo doc

[Li-Qingyun]

Error info:

❯ PYTHONPATH='.'$PYTHONPATH python quick_run.py
╭─────────────────────────────── Traceback (most recent call last) ────────────────────────────────╮
│ /home/lqy/Desktop/mmediting/quick_run.py:3 in <module>                                           │
│                                                                                                  │
│    1 import mmcv                                                                                 │
│    2 from mmedit.apis import init_model, restoration_inference                                   │
│ ❱  3 from mmedit.engine.misc import tensor2img                                                   │
│    4                                                                                             │
│    5 config = 'configs/esrgan/esrgan_x4c64b23g32_1xb16-400k_div2k.py'                            │
│    6 checkpoint = 'https://download.openmmlab.com/mmediting/restorers/esrgan/esrgan_x4c64b23g    │
╰──────────────────────────────────────────────────────────────────────────────────────────────────╯

[LeoXing1996]

@Li-Qingyun, thanks for your contribution. This PR LGTM!

[Li-Qingyun]

@LeoXing1996 Thank you for your maintenance! The MMEdit is Great!

[FRILLIC]

Where is the restoration_inference? I cannot import name 'restoration_inference' from 'mmagic.apis'.