[Bugfix] Fix padding bug in 12Hz tokenizer ConvTranspose1d decode

[linyueqian]

Purpose

Fix padding bugs in the 12Hz speech tokenizer decoder (Qwen3TTSTokenizerV2CausalTransConvNet), syncing with the upstream fix from QwenLM/Qwen3-TTS@5f8581d.

Two bugs in __init__ and forward:

1. Chained assignment bug: self.right_pad = pad = self.left_pad is a chained assignment that sets both pad and self.right_pad to self.left_pad, making them always equal. The intended behavior is left_pad = 0 and right_pad = kernel_size - stride.
2. Slicing bug: The old forward trimmed from both left and right unconditionally. Additionally, if right_pad is 0, hidden_state[..., :-0] returns an empty tensor (Python gotcha: x[:-0] == x[:0]). The fix only trims from the right and guards against right_pad == 0.

Test Plan

python examples/offline_inference/qwen3_tts/end2end.py --query-type CustomVoice

Test Result

Offline inference with Qwen3-TTS-12Hz-1.7B-CustomVoice completed successfully, producing valid audio output (shape=(247680,), sr=24000, ~10.3s).
output_0_94945aa2-880b-4966-8a44-a52f3147e11d.wav

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[Copilot]

Pull request overview

This pull request fixes two critical bugs in the Qwen3-TTS 12Hz tokenizer's ConvTranspose1d decoder (Qwen3TTSTokenizerV2CausalTransConvNet), syncing with an upstream fix from the official QwenLM/Qwen3-TTS repository.

Changes:

• Fixed chained assignment bug that incorrectly set left_pad and right_pad to the same value
• Fixed slicing bug that trimmed from both sides and caused empty tensor returns when right_pad == 0

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