Add environment variable to opt out of #10302 (forced disablement of …

[comfy-ovum]

…cudnn for all AMD users)

To offset the substantial effects of #10302, this PR provides (and informs the user of) an environment variable that can be set to nullify the unilateral decision made in #10302 to disable cudNN for all AMD users.

It simply employs the standard pattern for such things:

        torch.backends.cudnn.enabled = os.environ.get("TORCH_AMD_CUDNN_ENABLED", "0").strip().lower() not in {
            "0", "off", "false", "disable", "disabled", "no"}
        if not torch.backends.cudnn.enabled:
            logging.info(
                "ComfyUI has set torch.backends.cudnn.enabled to False for better AMD performance. Set environment var TORCH_AMD_CUDDNN_ENABLED=1 to enable it again.")

Should #10302 be later removed it is still a useful additional to enhance configurability for AMD users.

[alexheretic]

I think we still need the is_amd() check here, the following nested logic applies only to amd cards.