Store buffer without offset in GpuForceReduction::Impl::baseForce_

Store buffer without offset in GpuForceReduction::Impl::baseForce_

- Store the pointer to the beginning of the array and apply offset to
it when invoking the kernel. Offsets to other arrays are applied at
the invocation time, so it makes sense to do the same with baseForce_.
- This also avoid storing pointers to the middle of the device buffer,
which is not a good fit for OpenCL and SYCL.

This change is preparation for adding a SYCL implementation of GPU
update (Issue #3932).