[BUG] fix moe benchmark when bs*seq is small

[yiakwy-xpu-ml-framework-team]

Motivation

In "benchmark_deepseekv3_moe_align_blocks.py" change block_size 3 and run calculate_diff(batch_size=1, seq_len=4). The test will fail.

This is because expert_ids for cuda is not zero initialized and contain junk data.

Modifications

After fix, the script can run smoothly for

• calculate_diff(batch_size=4, seq_len=1024)
• calculate_diff(batch_size=1, seq_len=4)

Checklist

• Format your code according to the Code Formatting with Pre-Commit.
• Add unit tests as outlined in the Running Unit Tests.
• Update documentation / docstrings / example tutorials as needed, according to Writing Documentation.
• Provide throughput / latency benchmark results and accuracy evaluation results as needed, according to Benchmark and Profiling and Accuracy Results.
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