Merge branch 'hongzh/sell-cuda' into 'main'

SELL-based SpMV

See merge request petsc/petsc!3428

Add more tests

Add the load-balancing kernel for MatMultAdd_SeqSELL and fine tune the heuristic

Kernel7 is significantly slower than kernel9x for the following two cases:
- nrows is too small. Kernel7 uses 2 threa

Add the load-balancing kernel for MatMultAdd_SeqSELL and fine tune the heuristic

Kernel7 is significantly slower than kernel9x for the following two cases:
- nrows is too small. Kernel7 uses 2 threads per row (assuming sliceheight=16), it does not fully utilize the GPU if nrows < 100K.
- maxslicewidth is too big.

Thanks-to: Peng Wang <penwang@nvidia.com>

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