[BNB] Fix test_moving_to_cpu_throws_warning

[SunMarc]

What does this PR do?

Fixes tests/quantization/bnb/test_mixed_int8.py::SlowBnb8bitTests::test_moving_to_cpu_throws_warning due to this PR.
The issue is that since we used dispatch_model, when we load a 8-bit model, hooks will be attached to it (this is not the case for 4-bit). Due to that, pipeline_is_sequentially_offloaded is True -> is_offloaded also True. Thus, we didn't log the following :

            if (
                module.dtype == torch.float16
                and str(device) in ["cpu"]
                and not silence_dtype_warnings
                and not is_offloaded
            ):
                logger.warning(
                    "Pipelines loaded with `dtype=torch.float16` cannot run with `cpu` device. It"
                    " is not recommended to move them to `cpu` as running them will fail. Please make"
                    " sure to use an accelerator to run the pipeline in inference, due to the lack of"
                    " support for`float16` operations on this device in PyTorch. Please, remove the"
                    " `torch_dtype=torch.float16` argument, or use another device for inference."
                )

Traceback of the test before this PR

================================ short test summary info ================================
FAILED tests/quantization/bnb/test_mixed_int8.py::SlowBnb8bitTests::test_moving_to_cpu_throws_warning - assert 'Pipelines loaded with `dtype=torch.float16`' in "The module 'SD3Transformer2DModel' has been loaded in `bitsandbytes` 8bit and moving it to cpu via `.to()` is not supported. Module is still on cuda:0.\n"
 +  where "The module 'SD3Transformer2DModel' has been loaded in `bitsandbytes` 8bit and moving it to cpu via `.to()` is not supported. Module is still on cuda:0.\n" = captured: The module 'SD3Transformer2DModel' has been loaded in `bitsandbytes` 8bit and moving it to cpu via `.to()` is not supported. Module is still on cuda:0.\n\n.out

[HuggingFaceDocBuilderDev]

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[sayakpaul]

Thank you!

[sayakpaul]

Failing test is unrelated.