【Hackathon 9th No.39】Unit test for moe_expert_ffn_wint2

[cloudforge1]

Add unit tests for the wint2 quantized MoE expert FFN op.

Source: look for moe_expert_ffn_wint2 in custom_ops/gpu_ops/
Registration: custom_ops/gpu_ops/cpp_extensions.cc
Test file: tests/operators/test_moe_expert_ffn_wint2.py

This is a fused MoE FFN with wint2 quantized weights. Reference test: test_fused_moe.py. Cover different expert counts and hidden sizes.

Branch: task/039-moe-expert-ffn-wint2-test