make torch import in setup optional

[Qubitium]

Try to fix #1729

[jiqing-feng]

Got this error when pip install -e . on XPU

Obtaining file:///home/sdp/jiqing/GPTQModel
  Preparing metadata (setup.py) ... error
  error: subprocess-exited-with-error

  × python setup.py egg_info did not run successfully.
  │ exit code: 1
  ╰─> [12 lines of output]
      Traceback (most recent call last):
        File "<string>", line 2, in <module>
        File "<pip-setuptools-caller>", line 35, in <module>
        File "/home/sdp/jiqing/GPTQModel/setup.py", line 356, in <module>
          cpp_ext.CUDAExtension(
        File "/usr/local/lib/python3.10/dist-packages/torch/utils/cpp_extension.py", line 1337, in CUDAExtension
          library_dirs += library_paths(device_type="cuda")
        File "/usr/local/lib/python3.10/dist-packages/torch/utils/cpp_extension.py", line 1548, in library_paths
          if (not os.path.exists(_join_cuda_home(lib_dir)) and
        File "/usr/local/lib/python3.10/dist-packages/torch/utils/cpp_extension.py", line 2982, in _join_cuda_home
          raise OSError('CUDA_HOME environment variable is not set. '
      OSError: CUDA_HOME environment variable is not set. Please set it to your CUDA install root.
      [end of output]

  note: This error originates from a subprocess, and is likely not a problem with pip.
error: metadata-generation-failed

× Encountered error while generating package metadata.
╰─> See above for output.

note: This is an issue with the package mentioned above, not pip.
hint: See above for details.

[Qubitium]

Still buggy. We need to disable all nvcc/rocm kernel cpp_extension compliation when cuda is not found (xpu/cpu env).