text: Title: [Feature] generate till it reaches model's context length
  Body: ### Checklist
  
  - [ ] 1. If the issue you raised is not a feature but a question, please raise a discussion at https://github.com/sgl-project/sglang/discussions/new/choose Otherwise, it will be closed.
  - [ ] 2. Please use English, otherwise it will be closed.
  
  ### Motivation
  
  By default, sglang uses 128 tokens as the max_new_tokens even if None is given. Is there a way to specify that the model generated till it reaches it max model length? I have a different sequences of generation using sglang frontend language, so I cant keep track of the input lengths etc. I cant use one single max_new_tokens for different generations. If I put the max_new_tokens hardcoded, the input length is being restricted.
  
  Let me know if I can help implemenatation of such a feature, it would be very helpful. I am thinking of an implementation that takes the current input token length and mas token length to calculate max_new_tokens?
  Please let me know if this is valid concern.
  
  ### Related resources
  
  _No response_