Remove forward_backward batch cap from 256k Qwen3.8-27B definition

[micahtyong]

Summary

Drops MAX_FORWARD_BACKWARD_BATCH = 1 from qwen3_8_27b_miles_lora_256k.py so the engine uses its default (uncapped) coalescing: a step's 128 SDK chunks become one forward_backward op with batch 128 instead of 128 single-datum ops.

The cap was added in #39 as a guard against a suspected process-group wedge from long collective chains across both nodes. A 3-step probe on the matched 2-node TP2×CP8 topology with the cap removed ran clean (forward_backward_calls = 1/step, mean_batch = 128, no NCCL timeout/wedge/OOM), so the guard isn't needed. Performance is neutral: trainer-side forward_backward_s ≈ 2256 s in both configs; client train time 2470 s vs 2404 s capped over a 3-step sample.

Probe: https://wandb.ai/modal-labs/miles-lora-longcontext/runs/lilo-27b-256k-2n-nocap-3step-1790117905-1790117910

Link to Devin session: https://modal.devinenterprise.com/sessions/d017b320e7e1430aadae6c51e49c3029
Open in Devin Desktop: https://modal.devinenterprise.com/desktop/session/d017b320e7e1430aadae6c51e49c3029?variant=devin
Requested by: @micahtyong

[devin-ai-integration]

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