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* implicitly skip any test that implicitly uses CUDA on a non-CUDA box
* add a `requires_cuda` fixture

This PR adds initial FSDP support for training QLoRA models. It enables basic FSDP and CPU Offload support, with low memory training via FSDP.sync_module_states option unsupported.

This PR builds off of #840 commit 8278fca and BNB FSDP by @TimDettmers and @Titus-von-Koeller.

An example of using this PR to finetune QLoRA models with FSDP can be found in the demo repo: AnswerDotAi/fsdp_qlora.

* Minimal changes for fp32 4bit storage from BNB commit 8278fca

* Params4bit with selectable storage dtype

* possible fix for double quantizing linear weight & quant storage dtype

* minor fixes in Params4bit for peft tests

* remove redundant

* add float16

* update test

* Remove float16 quant cast as there are fp32, bf16, & fp16 quant kernels

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Co-authored-by: Kerem Turgutlu <keremturgutlu@gmail.com>