Summary

This PR improves CosyVoice2/3 streaming TTS + vLLM + TensorRT Flow inference in two commits. Both are backward-compatible: no breaking API changes, no new environment variables in cosyvoice/, and CosyVoice1 / non-streaming paths are untouched.

Benchmark and test details: #1892

Commit 1 — perf(inference): reduce streaming latency and optimize TRT inference

Streaming (CosyVoice2/3):

• Replace fixed time.sleep(0.1) polling with threading.Condition wake-ups so the main thread proceeds as soon as tokens arrive (~100ms polling waste removed).
• Override llm_job() to notify() after each token and on completion.
• Fix a bug: use local hop_len in the streaming loop instead of mutating self.token_hop_len, so repeated streaming requests reset hop correctly.
• Add first_token_hop_len as a class constant (currently 25, aligned with training chunk_size); trigger threshold unchanged vs upstream.

TensorRT (Flow DiT):

• Reuse one TRT execution context across all Euler steps in solve_euler() (was acquire/release per step).
• Use CUDA stream wait_stream() instead of device-wide synchronize() for finer overlap with vLLM.
• Fix TrtContextWrapper to store a real torch.cuda.Stream object.

Files: cli/model.py, flow/flow_matching.py, utils/common.py

Commit 2 — feat(inference): optional bucketed TensorRT engines for Flow DiT

Motivation: Default single TRT plan is built for max seq_len=3000. Short streaming chunks still run on a large engine, increasing latency and VRAM per context.

Changes:

• Add opt-in trt_bucket parameter to AutoModel / CosyVoice* / load_trt() (default False).
• Four bucket engines (256 / 768 / 1536 / 3000 mel frames), auto-built under {model_dir}/trt_bucket_plans/ when plans are missing.
• Introduce TrtBucketedContextWrapper for seq_len-aware routing via acquire_estimator(seq_len=...).
• Prefer flow.decoder.estimator.fp32.optimize.onnx, fallback to official fp32.onnx.
• Fix get_trt_kwargs() to use 6 ONNX inputs (x, mask, mu, t, spks, cond) with per-bucket profiles.

Files: utils/file_utils.py, utils/common.py, cli/model.py, cli/cosyvoice.py, flow/flow_matching.py

Compatibility

Test plan

• CosyVoice3 + load_vllm=True + load_trt=True, stream=True: first-packet latency vs upstream
• stream=False regression
• Multiple consecutive streaming requests: hop length resets correctly
• trt_bucket=False: single-plan path unchanged
• trt_bucket=True + optimize.onnx: auto-build 4 bucket plans on first run
• Short vs long utterances: smaller buckets used for short seq_len

Review guide

Use the Commits tab to review each commit independently:

1. Streaming + TRT hot-path optimizations (3 files)
2. Optional bucket TRT feature (5 files, opt-in)