Add CPU implementation for `torch._int_mm` (s8*s8->s32) (#121792)
Fixes #121647
**Description**
Currently, the op `torch._int_mm` only supports CUDA device. This PR adds CPU implementation for it.
Besides the request from the issue, this op may also be useful for planned CPU implementations of [LLM.int8()](https://arxiv.org/abs/2208.07339) in [Bitsandbytes](https://github.com/TimDettmers/bitsandbytes).
The implementation prefers mkldnn (oneDNN) kernels. If mkldnn is not available, a reference implementation with nested for loops is used.
**Test plan**
`python test/test_linalg.py -k test__int_mm_cpu`
Pull Request resolved: https://github.com/pytorch/pytorch/pull/121792
Approved by: https://github.com/jgong5, https://github.com/lezcano
