[UR] Fix creation of context with parent device and its sub-devices

[againull]

This use case seems to be allowed. For example, according to: https://intel.github.io/llvm/MultiTileCardWithLevelZero.html#context "Both root-devices and sub-devices can be within single context, but they all should be of the same SYCL platform."

Before change CollectDevicesAndSubDevices was resulting in duplicate devices being returned which leads to an error in

llvm/unified-runtime/source/common/ur_pool_manager.hpp

Lines 178 to 179 in 6af08fe

	if (!descToPoolMap.try_emplace(desc, std::move(hPool)).second) {
	UR_LOG(ERR, "Pool for pool descriptor: {}, already exists", desc);

because of the duplicate pool descriptors.