Add linalg.batch_matvec named op

Add linalg.batch_matvec named op

    Similarly to batch_mat vec outer most dim is a batching dim
    and this op does |b| matrix-vector-products :
    C[b, i] = sum_k(A[b, i, k] * B[b, k])

Reviewed By: rsuderman

Differential Revision: https://reviews.llvm.org/D104739