Fix for gemm with zero strides plus add unit test for torch.mm

[fmassa]

• Solves blas fails for gemm with zero strides #58
• Also solves the issue of gemm when the resulting tensor has zero stride (as in Problem with pointwise math when output tensor is zero-strided with correct size #289)
• Adds unit test for torch.mm, which was missing

[soumith]

Thanks franceyyy!