Upgrade accelerate version

Upgrade accelerate to fix the following error:

```
ERROR: Cannot install -r requirements.txt (line 1) and accelerate==0.20.3 because these package versions have conflicting dependencies.

The conflict is caused by:
The user requested accelerate==0.20.3
peft 0.6.0.dev0 depends on accelerate>=0.21.0
```

requirements.txt

@@ -4,4 +4,4 @@ transformers==4.30.2
 torch==2.0.0
 bitsandbytes==0.39.1
 loralib==0.1.1
-accelerate==0.20.3
+accelerate==0.21.0