Remove degenerate batch dimensions form batch dot

The way things are set up today this specific optimization isn't particularly
important, but I want to implement a follow-on optimization in
BatchDotSimplification to transform (non-degenerate) batch GEMV operations into
GEMM which I'm expecting to help us a bit.

This would normally be in the algebraic simplifier, but we want to fixpoint this
pass before we run DotDecomposer.  This will become more important when we
implement the (non-degenerate) batch GEMV operations -> GEMM transform.

PiperOrigin-RevId: 196314230