[sharktank] Remove 'torch' from deps and warn instead

Instead of enforcing the installation for 'torch' as a dependency, error
if 'torch' cannot be imported and point the user to how to install.

docs/user_guide.md

@@ -34,14 +34,18 @@ Setup your Python environment with the following commands:
 # Set up a virtual environment to isolate packages from other envs.
 python3.11 -m venv 3.11.venv
 source 3.11.venv/bin/activate
+```
+
+## Install SHARK and its dependencies
+
+Before installing `shark-ai` install a torch version that fulfills your needs.
 
-# Optional: faster installation of torch with just CPU support.
+```bash
+# Fast installation of torch with just CPU support.
 # See other options at https://pytorch.org/get-started/locally/
 pip install torch torchvision torchaudio --index-url https://download.pytorch.org/whl/cpu
 ```
 
-## Install SHARK and its dependencies
-
 ```bash
 pip install shark-ai[apps]
 ```

sharktank/requirements.txt

@@ -9,10 +9,6 @@ huggingface-hub==0.22.2
 transformers==4.40.0
 datasets
 
-# It is expected that you have installed a PyTorch version/variant specific
-# to your needs, so we only include a minimum version spec.
-torch>=2.3.0
-
 # Serving deps.
 fastapi>=0.112.2
 uvicorn>=0.30.6

sharktank/sharktank/__init__.py

@@ -3,3 +3,13 @@
 # Licensed under the Apache License v2.0 with LLVM Exceptions.
 # See https://llvm.org/LICENSE.txt for license information.
 # SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+
+import importlib.util
+
+msg = """No module named 'torch'. Follow https://pytorch.org/get-started/locally/#start-locally to install 'torch'.
+For example, on Linux to install with CPU support run:
+  pip3 install torch --index-url https://download.pytorch.org/whl/cpu
+"""
+
+if spec := importlib.util.find_spec("torch") is None:
+    raise ModuleNotFoundError(msg)