Remaining pieces for upstreaming

[sunggg]

These are prerequisite for making mlc-serve an independent package.

• Mixtral support @vinx13
• vLLM v2 kernel @vinx13
• Misc changes in core.py for mlc-serve-specific artifact dump @sunggg
• Batched model support for split + rotary fusion (mlc_llm/transform/fuse_split_rotary_embedding.py). This one depends on a hack to TVM