QWen VL Follow-up Fixes

[merrymercy]

There are still some remaining items to finish the QWen VL support.

1. There might be a small bug in the image process. The tests in CI will trigger this error sometimes. https://github.com/sgl-project/sglang/actions/runs/11598427301/job/32294256076#step:4:778
2. This mrope_position_delta is very model specific and seems only related to multi modal.

   sglang/python/sglang/srt/managers/schedule_batch.py

   Line 255 in efbc116

   self.mrope_position_delta = [] # use mutable object

   We should move this out of class Req. We can pay some costs to recompute it every time if that is possible. Otherwise, we can move that to Class ImageInputs.