Make the Csr::automatical strategy work with gko::Executor

[thoasm]

Currently, we need either a HipExecutor or a CudaExecutor to create the Csr::automatical strategy.
It would be a lot easier to have a constructor that just takes a gko::Executor, so we could use it with any executor.

One way of implementing that is to remove the constructors that take HipExecutor and CudaExecutor and replace it with one that just takes Executor, which uses dynamic_cast to check for CUDA and HIP executors, while also working for different executors.

However, this might break interface, so we have to discuss on that.

[upsj]

This is also slightly related to the "double-destroy" segfaults we had with #418, since the automatical strategy is default-constructed with a CudaExecutor

[tcojean]

The default CudaExecutor was here before, because automatical in the previous releases was purely a Cuda strategy. Changing this would definitely break interface. We can consider this a bugfix maybe and then justify that change.

On the other hand, the change proposed here should be fine, we would have the same constructor with the same parameters which should do the same thing as currently given the same argument.

[tcojean]

#969 implemented a non-interface breaking way of achieving the same result. But Csr::automatical and strategies in general still need to be fixed.

[upsj]

Sounds like the remaining work is covered by #320, I think this can be closed