Don't manage `PYTORCH_NVML_BASED_CUDA_CHECK` when calling `accelerate.utils.imports.is_cuda_available()` (#2524)
* Don't manage PYTORCH_NVML_BASED_CUDA_CHECK
PYTORCH_NVML_BASED_CUDA_CHECK will use an NVML-based check when
determining how many devices are available. That's useful for preventing
CUDA initialization when doing that check (or calling
`torch.cuda.is_available()`). Instead of manipulating that env var, one
can call the torch utility `_device_count_nvml` directly preventing the
manipulation of the env var.
* Uses env var instead of private torch function
* Fixes flake8 check