[XLA:GPU] Add a Gemv rewriter pass to convert gemv to gemm with a trivial dimension
[XLA:GPU] Add a Gemv rewriter pass to convert gemv to gemm with a trivial dimension

We need this pass since GemmFusion only accepts gemms. We should run this pass before GemmFusion. After GemmFusion, we should use AlgebraicSimplifier to remove trivial dimension from gemms that are not fused by GemmFusion.
