[mlir] [VectorOps] generalized vector.contract semantics

Summary:
Previously, vector.contract did not allow an empty set of
free or batch dimensions (K = 0) which defines a basic
reduction into a scalar (like a dot product). This CL
relaxes that restriction. Also adds constraints on
element type of operands and results. With tests.

Reviewers: nicolasvasilache, andydavis1, rriddle

Reviewed By: andydavis1

Subscribers: merge_guards_bot, mehdi_amini, rriddle, jpienaar, burmako, shauheen, antiagainst, nicolasvasilache, arpith-jacob, mgester, lucyrfox, liufengdb, Joonsoo, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D74014