[TOPI] Add handwritten matvec for dynamic cases

[tkonolige]

Add a handwritten matrix-vector multiplication implementation for dynamic cases on cpu. This avoids crashing when a dynamic shape is present. Only supports vector LHS and float32 for now.

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[tvm-bot]

Thanks for contributing to TVM! Please refer to the contributing guidelines https://tvm.apache.org/docs/contribute/ for useful information and tips. Please request code reviews from Reviewers by @-ing them in a comment.

• No users to tag found in teams: topi _{See #10317 for details}

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[jwfromm]

This all makes sense to me, I left a few nitpicks but otherwise should be good to go.

[tkonolige]

@jwfromm a change: the dynamic kernel usage is predicated on it being a matrix-vector multiply instead of asserting within the kernel. Apparently there are some other code paths that do dynamic matmul that I broke.

[jwfromm]

Yeah that makes perfect sense to me, good change.

[tkonolige]

@tvm-bot rerun

[michalpiszczek]

@tvm-bot rerun

[michalpiszczek]

@tvm-bot rerun

[junrushao]

LGTM!