[tests] make test device-agnostic

[faaany]

What does this PR do?

Make test_from_pretrained_disk_offload_derived_to_base_model and test_from_pretrained_disk_offload_task_model tests work not only on GPU device, but also other accelerator devices.

Results on my XPU device:

========================================================== PASSES ==========================================================
__________________________ ModelUtilsTest.test_from_pretrained_disk_offload_derived_to_base_model __________________________
---------------------------------------------------- Captured log call -----------------------------------------------------
WARNING  root:big_modeling.py:435 Some parameters are on the meta device device because they were offloaded to the disk and cpu.
WARNING  root:big_modeling.py:435 Some parameters are on the meta device device because they were offloaded to the disk and cpu.
================================================= short test summary info ==================================================
PASSED tests/test_modeling_utils.py::ModelUtilsTest::test_from_pretrained_disk_offload_derived_to_base_model
============================================== 1 passed, 3 warnings in 1.46s ===============================================

@amyeroberts and @ArthurZucker

[amyeroberts]

Another great improvement to our tests - thanks!