support o-series models

[yulin-li]

I got the following error when trying to use o-series models. The max_tokens is change to max_completion_tokens for o-series models.

{
"error": {
"message": "Unsupported parameter: 'max_tokens' is not supported with this model. Use 'max_completion_tokens' instead.",
"type": "invalid_request_error",
"param": "max_tokens",
"code": "unsupported_parameter"
}