[sparse] test out-of-bound csr/coo indices on GPU

[jakevdp]

I found that the coo lowering rules don't offer any good avenue for working with padded sparse representations because (1) if out-of-bound indices are present it leads to out-of-bound memory access, and (2) if duplicate indices are present, cusparse is inconsistent on whether it treats those duplicated indices as summed, or as overwriting each other.

For this reason, I don't think there's any way to devise any efficient padded representation that will work correctly with all lowering rules.

The solution is to add another static flag that tracks whether the matrix is padded, and only lower to GPU when that is known to be false.