Saving some VRAM. (#2790)
* Saving some VRAM.
- 8B on 4xL4 attention=flashdecoding . Before 4.28GB left, After 4.32GB
left, so 400MB saved.
- Effect not as visible on attention=flashinfer and n_shard=1. I suspect
it's linked to the torch allocator.
* Adding assertion.
