Pin torch to exact cu128 build to match torchaudio

Without an explicit torch pin, pip resolved a torch build that didn't
ship libcudart.so.12, so torchaudio's compiled CUDA extension failed
to load at runtime (RUNTIME_ERROR, not build error).

requirements.txt

@@ -21,5 +21,6 @@ torchdiffeq
 open_clip_torch
 git+https://github.com/descriptinc/audiotools
 --extra-index-url https://download.pytorch.org/whl/cu128
+torch==2.11.0+cu128
 torchaudio==2.11.0+cu128
 mmcv-lite==2.2.0