Fix grammar-based sampling issue in server

[krasserm]

This PR fixes a reported issue with grammar-based sampling in server.cpp. Problem was a different scope of grammar and parsed_grammar.

[SlyEcho]

curl --request POST   --url http://localhost:8080/completion   --header 'Content-Type: application/json'   --data '{
        "prompt": "Schedule a birthday party on Aug 14th 2023 at 8pm.",
        "temperature": 0.1,
        "n_predict": 128,
        "grammar": "space ::= \" \"?\nstring ::=  \"\\\"\" (\n        [^\"\\\\] |\n        \"\\\\\" ([\"\\\\/bfnrt] | \"u\" [0-9a-fA-F] [0-9a-fA-F] [0-9a-fA-F] [0-9a-fA-F])\n      )* \"\\\"\" space \narguments ::= \"{\" space \"\\\"date\\\"\" space \":\" space string \",\" space \"\\\"time\\\"\" space \":\" space string \",\" space \"\\\"title\\\"\" space \":\" space string \"}\" space\nfunction ::= \"\\\"create_event\\\"\"\nroot ::= \"{\" space \"\\\"arguments\\\"\" space \":\" space arguments \",\" space \"\\\"function\\\"\" space \":\" space function \"}\" space\n"
}' | jq '.content|fromjson'

response:

{
  "arguments": {
    "date": "2023-08-14",
    "time": "20:00:00",
    "title": "Birthday Party"
  },
  "function": "create_event"
}