Replace CPU-bound Q-value monitoring with GPU-resident qvalue_stats and
qvalue_divergence kernels from GpuTrainingGuard. The two readback sites
(estimate_avg_q_value_with_early_stopping's mean_all().to_scalar() and
log_q_values' to_vec2()) are now bypassed when the GPU training guard is
active. CPU fallback path preserved for non-CUDA builds and guard-absent
scenarios.
Changes:
- Add DQN::log_q_values_from_stats() accepting pre-computed GPU stats
- Add DQNAgentType::log_q_values_from_stats() delegate
- Replace Q-estimation in train_step_single_batch with GPU kernel path
- Replace Q-estimation in train_step_with_accumulation with GPU kernel path
- Import IndexOp trait for Tensor::i() in trainer.rs
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
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