[GRPO] add chunked grpo streaming over vocab

[kashif]

Summary

This PR fixes the chunked GRPO loss to compute only selected-token log-probs by streaming over the vocab dimension. This reduces peak memory for the fused-linear chunked path and preserves the existing high-level fused-linear API.

We also fixes the luspo reduction in the chunked path to match TRL exactly, and tightens the torch.compile boundary so we only compile the pure loss computation instead of compiling through the closure that calls torch.autograd.grad.

So now the two implementations are correctly implementing the trade-offs of their design.

Testing Done

• Hardware Type:
• run make test to ensure correctness
• run make checkstyle to ensure code style
• run make test-convergence to ensure convergence

[kashif]

cc @vaibhavjindal for your review