Fix missing diff[i] in legacy contrastive loss GPU backward

[Chessing234]

Summary

The GPU backward kernel (CLLBackward) for ContrastiveLossLayer produces wrong gradients when legacy_version=true for dissimilar pairs.

Root cause: The CPU backward computes the gradient via caffe_cpu_axpby:

beta = -alpha;
caffe_cpu_axpby(channels, beta, diff_.cpu_data() + (j*channels), ...);
// result: bout[d] = -alpha * diff[d]

The GPU kernel sets bottom_diff[i] = beta = -alpha directly, omitting the multiplication by diff[i]:

beta = -alpha;          // missing: * diff[i]
bottom_diff[i] = beta;  // gives -alpha instead of -alpha * diff[i]

The non-legacy GPU path correctly absorbs diff[i] into beta:

beta = -alpha * mdist / (dist + 1e-4) * diff[i];  // correct

Fix: Change beta = -alpha to beta = -alpha * diff[i] to match the CPU implementation.

Test plan

• Run contrastive loss layer tests with legacy_version=true on GPU
• Verify CPU and GPU backward outputs match for dissimilar pairs

🤖 Generated with Claude Code