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.

[sfinktah]

(quickly double checks)... it is in there, oh wait... hmm.... how did that happen! Fixed now.

That reminds me that the RDNA2 cut-off point is somewhat arbitary, but not my code. RDNA2 VAE decoding certainly benefits just as much as RDNA3. Not sure how it is when you aren't using cobbled together Windows drivers though.

[alexheretic]

lgtm, much nicer to have env control of this rather than maintaining patches 👍

[sfinktah]

lgtm, much nicer to have env control of this rather than maintaining patches 👍

ngl, hopes are not high.