Refactor Request: Grammar should be put as string in the example scripts to help run directly

```python
import torch
from transformers import AutoModelForCausalLM, AutoTokenizer
from transformers_cfg.grammar_utils import IncrementalGrammarConstraint
from transformers_cfg.generation.logits_process import GrammarConstrainedLogitsProcessor

if __name__ == "__main__":

device = torch.device("cuda" if torch.cuda.is_available() else "cpu")
print(f"Using device: {device}")

model_id = "mistralai/Mistral-7B-v0.1"

# Load model and tokenizer
tokenizer = AutoTokenizer.from_pretrained(model_id)
tokenizer.pad_token = tokenizer.eos_token

model = AutoModelForCausalLM.from_pretrained(model_id).to(
device
) # Load model to defined device
model.generation_config.pad_token_id = model.generation_config.eos_token_id

model = AutoModelForCausalLM.from_pretrained(model_id).to(
device
) # Load model to defined device
model.generation_config.pad_token_id = model.generation_config.eos_token_id

grammar_str = """
# Grammar for subset of JSON
# String doesn't support unicode and escape yet
# If you don't need to generate unicode and escape, you can use this grammar
# We are working to support unicode and escape

root ::= object

object ::= "{" ws ( string ":" ws value ("," ws string ":" ws value)* )? "}"

value ::= object | array | string | number | ("true" | "false" | "null") ws

array ::= "[" ws ( value ("," ws value)* )? "]" ws

string ::= "\"" [ \t!#-\[\]-~]* "\"" ws

number ::= ("-"? ([0-9] | [1-9] [0-9]*)) ("." [0-9]+)? ([eE] [-+]? [0-9]+)? ws

ws ::= ([ \t\n] ws)?
"""
grammar = IncrementalGrammarConstraint(grammar_str, "root", tokenizer)
grammar_processor = GrammarConstrainedLogitsProcessor(grammar)

# Generate
prefix1 = "This is a valid json string for http request:"
prefix2 = "This is a valid json string for shopping cart:"
input_ids = tokenizer(
[prefix1, prefix2], add_special_tokens=False, return_tensors="pt", padding=True
)["input_ids"].to(
device
) # Move input_ids to the same device as model

output = model.generate(
input_ids,
do_sample=False,
max_new_tokens=60,
logits_processor=[grammar_processor],
repetition_penalty=1.1,
num_return_sequences=1,
)
# decode output
generations = tokenizer.batch_decode(output, skip_special_tokens=True)
print(generations)

"""
'This is a valid json string for http request:{ "request": { "method": "GET", "headers": [], "content": "Content","type": "application" }}
'This is a valid json string for shopping cart:This is a valid json string for shopping cart:{ "name": "MyCart", "price": 0, "value": 1 }
"""
```