fix(contrib): call torch.cuda.is_available() in FP16_Optimizer guard

[andrewwhitecdw]

Summary

apex.contrib.optimizers.FP16_Optimizer.__init__ is supposed to fail fast with a clear SystemError("Cannot use fp16 without CUDA.") on machines without CUDA. The guard never fires, so on a CPU-only machine (or a build without the CUDA extensions) construction proceeds and blows up later with a confusing RuntimeError: FP16_Optimizer requires cuda extensions from deep in the initializer.

Root cause

if not torch.cuda.is_available:
    raise SystemError("Cannot use fp16 without CUDA.")

torch.cuda.is_available (without parentheses) is the bound method object, which is always truthy, so not torch.cuda.is_available is always False and the raise is dead code.

Fix

Call the function:

if not torch.cuda.is_available():
    raise SystemError("Cannot use fp16 without CUDA.")

One-line change; behavior on CUDA machines is unchanged (the guard still does not fire there).

Testing

On a CPU-only machine with an ephemeral env (uv run --with torch), using torch.optim.SGD as the wrapped optimizer:

• With the fix: FP16_Optimizer(opt, static_loss_scale=128.0) raises SystemError: Cannot use fp16 without CUDA. immediately — the intended behavior.
• With the fix stashed (unmodified master, 6424da3): the same call skips the guard and instead raises RuntimeError: FP16_Optimizer requires cuda extensions from line 64, confirming the guard was dead code.

⚠️ The CUDA-dependent test suites under tests/ (L0/L1) require a GPU and the compiled amp_C extensions, so they could not be run locally; this change does not touch any CUDA code path, but please rely on CI for the full suite.

Why existing tests missed it

The optimizer tests run exclusively on CUDA machines, where the guard is a no-op either way; nothing exercises the CPU-only error path.

[crcrpar]

looks good but I'm afraid I don't think the test is necessary

[andrewwhitecdw]

Closing per adversarial audit: branch contains off-scope or not-a-bug changes.

[andrewwhitecdw]

script closed on accident