Check whether `get_ipu_device` actually returns an IPU device

[giordano]

In some cases, e.g. if no IPU devices are available for some reasons (including misconfiguration of the current system), Poplar.get_ipu_device() may return a device, which is actually an IPU model, but then code may not work if using hardware IPU is important. We should check that the return device is an IPU and throw an error otherwise, we should be able to query the TargetType of the device with getTargetType