【Hackathon 9th No.51】Unit test for masked_per_token_quant

[cloudforge1]

Add unit tests for masked_per_token_quant.

⚠️ Important: Investigate before implementing

Three upstream tasks (PaddlePaddle#20, PaddlePaddle#35, PaddlePaddle#51) reference masked_per_token_quant. Tasks PaddlePaddle#20 and PaddlePaddle#35 are COMPLETED (PRs #4111 and #3867).

This task (PaddlePaddle#51) must cover something NOT already tested.

The op masked_per_token_quant does NOT appear directly in cpp_extensions.cc. Check:

• per_token_quant at cpp_extensions.cc:1283
• test_fused_masked_swiglu_quant.py for the fused version
• Python modules for possible pure-Python implementation

Source files to study:

• custom_ops/gpu_ops/cpp_extensions.cc — search per_token_quant
• tests/operators/test_fused_masked_swiglu_quant.py — reference for fused version
• tests/operators/test_dynamic_per_token_scaled_fp8_quant.py — related quant test

Test file: tests/operators/test_masked_per_token_quant.py

Branch: task/051-masked-per-token-quant-test