Modify GEMM to handle m=0, n=0, and k=0 cases.
Modify GEMM to handle m=0, n=0, and k=0 cases.

rotorliu/eigen,rotorliu/eigen,madlib/eigen_backup,madlib/eigen_backup,rotorliu/eigen,rotorliu/eigen,madlib/eigen_backup,madlib/eigen_backup