Fix DeepCompile ZeRO-1 grad target lifetime

[tohtana]

DeepCompile ZeRO-1 kept compile-time reduce target buffers alive into the optimizer step, causing backward gradient storage to overlap with optimizer temporaries. This PR fixes the issue by making DeepCompile ZeRO-1 reduce targets follow the normal step-local ZeRO partition gradient-buffer lifetime, instead of preserving cloned target storage from compile setup.

The actual code changes are:

• During compile initialization, register empty DeepCompile ZeRO-1 gradient targets, then bind them to the step-local flat ZeRO partition gradient buffer and per-parameter views when gradients are ready to synchronize.
• After ZeRO-1 builds the optimizer-facing fp32 gradient partition, release the DeepCompile registry references and clear reduce bucket storage after backward synchronization.
• Keep this branch limited to the flat-buffer lifetime fix, without the experimental direct-reduce path or allocator fallback behavior.

	Step 10-30 avg sec	Peak alloc GiB
Without this PR	0.858	43.594
Without this PR	0.859	39.366

Fine-tuning style training (8xH100, Qwen3-8B random weights, bs/GPU=1, seq=4096, GAS=1) showed only finite value losses for 1000 steps.

[chatgpt-codex-connector]

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