[OBCQ Bug Fix] Fallback to CPU if no CUDA

[Satrat]

tests.sparseml.transformers.obcq.test_obcq was failing on Jenkins due to CUDA not being available, this PR updates the OBCQ script to fall back to CPU if CUDA is unavailable. The model used in this test is very small, so it still runs quickly enough on CPU.

Also fixes a small bug in the perplexity calculations in the OBCQ script caused by the recent target_ids change

Testing

export CUDA_VISIBLE_DEVICES=[]
pytest tests/sparseml/transformers/obcq/test_obcq.py

Output:

WARNING    sparseml.transformers.sparsification.obcq.obcq:obcq.py:156 Requested cuda:0 but CUDA is not available, falling back to CPU
.....
tests/sparseml/transformers/obcq/test_obcq.py::test_obcq_tinystories[tests/sparseml/transformers/obcq/test_tiny.yaml] PASSED                                                                     [ 50%]
tests/sparseml/transformers/obcq/test_obcq.py::test_obcq_tinystories[tests/sparseml/transformers/obcq/test_tiny2.yaml] PASSED                                                                    [100%]