[ROCm][CI TG] refactor and fix deepep_moe test group

[divakar-amd]

Fix test_deepep_moe.py on ROCm and consolidate MoE test accuracy checks

1. Buffer reuse (low-latency DeepEP): The modular kernel and its DeepEP all2all buffer are now built once and reused across all token chunks instead of being re-created per chunk. Re-creating it per chunk re-initializes rocSHMEM, which only supports a single allocation per process on ROCm and aborts with "Unknown allocator type". The low-latency buffer is sized by max_tokens_per_rank/hidden_size (not chunk size), so one buffer is valid for every chunk — mirroring production, where the all2all handle is cached and reused (DeepEPLLAll2AllManager). Because the buffer is reused (and thus left dirty by the previous chunk), clean_low_latency_buffer is called before each dispatch to restore the zero-initialized regions the low-latency kernels require.

   • E0000h rocSHMEM Unknown allocator type SingleHeap@src/memory/single_heap.cpp:58 (BUILDKITE Error LINK for reference)

2. Accuracy check consolidation: ROCm's fp8 format (e4m3fnuz) rounds differently than NVIDIA's e4m3fn, so a few elements in the DeepEP MoE output can exceed a strict bit-for-bit tolerance even though the kernel is correct. Rather than add a bespoke comparison, this reuses the existing check_accuracy mismatch-fraction helper:

   (a.) Moved check_accuracy from test_ocp_mx_moe.py into the shared tests/kernels/moe/utils.py (already used by cuda MXFP4/MXFP8 paths for accuracy check). test_ocp_mx_moe.py now imports it; call sites unchanged.
   (b.) Re-use the above check_accuracy for test_deepep_moe.py. The base path uses percent=1.0 (equivalent to the previous assert_close); the relaxed tolerance is gated behind current_platform.is_fp8_fnuz().

Fixes: tests/kernels/moe/test_deepep_moe.py::test_low_latency_deep_ep_moe

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[claude]

Claude Code Review

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[tjtanaa]

LGTM now