fix(tada): run voice-prompt encode under torch.inference_mode (#955)

Encoder.eval() alone still builds an autograd graph because parameters
require grad by default. On 8GB GPUs that ballooned TADA encode VRAM far
past the model footprint (issue 890). Wrap the encode forward in
inference_mode and add a unit test that asserts the flag is set.

Co-authored-by: fooSynaptic <[email protected]>

backend/tests/test_hume_encode_inference_mode.py

@@ -0,0 +1,68 @@
+"""Ensure TADA voice-prompt encoding disables autograd (#890)."""
+
+from __future__ import annotations
+
+from dataclasses import dataclass
+from unittest.mock import AsyncMock
+
+import numpy as np
+import pytest
+import soundfile as sf
+import torch
+
+from backend.backends.hume_backend import HumeTadaBackend
+
+
+@dataclass
+class _FakeEncoderOutput:
+    emb: torch.Tensor
+
+
+class _GradTrackingEncoder:
+    """Raises unless called under torch.inference_mode()."""
+
+    def __init__(self) -> None:
+        self.called_under_inference_mode = False
+
+    def __call__(self, audio, text=None, sample_rate=None):
+        self.called_under_inference_mode = torch.is_inference_mode_enabled()
+        if not self.called_under_inference_mode:
+            raise AssertionError("encoder forward must run under inference_mode")
+        # Touch a requires_grad tensor the way Snake1d alpha would.
+        alpha = torch.nn.Parameter(torch.ones(1, device=audio.device))
+        _ = audio.mean() * alpha
+        return _FakeEncoderOutput(emb=torch.zeros(1, 4, device=audio.device))
+
+
+@pytest.mark.asyncio
+async def test_create_voice_prompt_runs_encoder_under_inference_mode(tmp_path, monkeypatch):
+    wav = tmp_path / "ref.wav"
+    sf.write(str(wav), np.zeros(24000, dtype=np.float32), 24000)
+
+    backend = HumeTadaBackend()
+    backend.model = object()  # mark loaded
+    backend.model_size = "1B"
+    backend._device = "cpu"
+    encoder = _GradTrackingEncoder()
+    backend.encoder = encoder
+
+    monkeypatch.setattr(backend, "load_model", AsyncMock(return_value=None))
+    monkeypatch.setattr(
+        "backend.backends.hume_backend.get_cached_voice_prompt",
+        lambda key: None,
+    )
+    monkeypatch.setattr(
+        "backend.backends.hume_backend.cache_voice_prompt",
+        lambda key, value: None,
+    )
+
+    prompt, from_cache = await backend.create_voice_prompt(
+        str(wav),
+        reference_text="hello world",
+        use_cache=False,
+    )
+
+    assert from_cache is False
+    assert encoder.called_under_inference_mode is True
+    assert isinstance(prompt["emb"], torch.Tensor)
+    assert prompt["emb"].device.type == "cpu"