Same dead-flag chain as FFLAG-011: GpuDqnTrainConfig.enable_gradient_vaccine
→ local vaccine_enabled → GpuDqnTrainer.enable_gradient_vaccine field.
Zero readers for self.enable_gradient_vaccine — the vaccine kernels
(vaccine_dot_kernel, vaccine_project_kernel) run unconditionally. Deleted
field + struct field + local + setter at fused_training, 3 sites total.