Cherry-pick key PRs from unity to support LLaMA/Vicuna optimization

[yelite]

This includes:

• https://github.com/apache/tvm/pull/15118
• https://github.com/apache/tvm/pull/15028
• https://github.com/apache/tvm/pull/15030
• https://github.com/apache/tvm/pull/15029

cc @sunggg

[tqchen]

we will do a rebase soon, so no cherry pick is needed

[tqchen]

the rebase is now completed