fix(deps): make torch/ML stack opt-in instead of a mandatory install

requirements-amd.txt/requirements-nvidia.txt were pulling a multi-GB
torch wheel by default even though nothing in synapse/ imports torch,
transformers, accelerate, bitsandbytes, or PySide6 - dead weight that
made the pip batch fragile (one failed download could take unrelated
base deps down with it on a slow connection). Split the unused ML/GUI
stack out of requirements-base.txt into a new opt-in requirements-ml.txt,
and dropped the torch lines from the AMD/NVIDIA overlays and generator.

Also: recreate the venv if it exists but pip is missing, instead of
silently reusing a half-built one (ensurepip can fail during venv
creation and leave an interpreter with no pip).

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

requirements-amd.txt

@@ -4,7 +4,6 @@
 
 -r requirements-base.txt
 
-# GPU Compute Stack
-torch
-torchaudio
-torchvision
+# Torch itself is opt-in — see requirements-ml.txt. The --index-url above is
+# what makes `pip install -r requirements-amd.txt -r requirements-ml.txt`
+# resolve torch as the ROCm build instead of CPU-only.