error handling for no torch

setup.py

@@ -1,9 +1,13 @@
 import os
 from pathlib import Path
 
-import torch
 from setuptools import find_packages, setup
-from torch.utils.cpp_extension import BuildExtension, CUDAExtension
+
+try:
+    import torch
+    from torch.utils.cpp_extension import BuildExtension, CUDAExtension
+except ModuleNotFoundError as e:
+    raise ModuleNotFoundError("No module named 'torch'. `torch` is required to install `grouped_gemm`.",) from e
 
 if os.environ.get("TORCH_CUDA_ARCH_LIST"):
     # Let PyTorch builder to choose device to target for.