Add torch to setup_requires & dynamic import to prevent import erro…

…rs when installing via pip (#514)

* dynamic import of torch to prevent import error when installing

* Update setup.py

Co-authored-by: Xintao <wxt1994@126.com>
Co-authored-by: Hans Brouwer <hans@brouwer.work>

setup.py

@@ -5,8 +5,6 @@
 import os
 import subprocess
 import time
-import torch
-from torch.utils.cpp_extension import BuildExtension, CppExtension, CUDAExtension
 
 version_file = 'basicsr/version.py'
 
@@ -117,6 +115,12 @@ def get_requirements(filename='requirements.txt'):
 if __name__ == '__main__':
     cuda_ext = os.getenv('BASICSR_EXT')  # whether compile cuda ext
     if cuda_ext == 'True':
+        try:
+            import torch
+            from torch.utils.cpp_extension import BuildExtension, CppExtension, CUDAExtension
+        except ImportError:
+            raise ImportError('Unable to import torch - torch is needed to build cuda extensions')
+
         ext_modules = [
             make_cuda_ext(
                 name='deform_conv_ext',
@@ -134,8 +138,10 @@ def get_requirements(filename='requirements.txt'):
                 sources=['src/upfirdn2d.cpp'],
                 sources_cuda=['src/upfirdn2d_kernel.cu']),
         ]
+        setup_kwargs = dict(cmdclass={'build_ext': BuildExtension})
     else:
         ext_modules = []
+        setup_kwargs = dict()
 
     write_version_py()
     setup(
@@ -159,8 +165,8 @@ def get_requirements(filename='requirements.txt'):
             'Programming Language :: Python :: 3.8',
         ],
         license='Apache License 2.0',
-        setup_requires=['cython', 'numpy'],
+        setup_requires=['cython', 'numpy', 'torch'],
         install_requires=get_requirements(),
         ext_modules=ext_modules,
-        cmdclass={'build_ext': BuildExtension},
-        zip_safe=False)
+        zip_safe=False,
+        **setup_kwargs)