Special case NonLazyTensor matmul in DiagLazyTensor
Special case NonLazyTensor matmul in DiagLazyTensor

We don't need to have this be lazy we can just do element-wise multiplication of the underlying tensors (w/ broadcasting).

Also uses `diag_embed` instead of `eye` for `evaluate`.

jrg365/gpytorch,jrg365/gpytorch,jrg365/gpytorch