Add a CPU version

[Xiaoyang-Rebecca]

• converted the splat function from c++ to python. Able to run the unit test of splat_2d in CPU. https://github.com/Xiaoyang-Rebecca/gangealing/blob/cpu/unit_tests/test_splat.py

• added --device "cpu" option and other places calling device='cuda

• [TODO] Need to support StyleGAN inferencing in cpu, but stylegan2/op only support cuda. Any suggestion? @wpeebles

nohup: ignoring input
No CUDA runtime is found, using CUDA_HOME='/usr'
Traceback (most recent call last):
  File "applications/propagate_to_images.py", line 20, in <module>
    from applications import base_eval_argparse, load_stn, determine_flips
  File "project-py-GANgealing/applications/__init__.py", line 3, in <module>
    from models import get_stn, ResnetClassifier
  File "project-py-GANgealing/models/__init__.py", line 3, in <module>
    from models.spatial_transformers.spatial_transformer import get_stn, ComposedSTN, SpatialTransformer
  File "project-py-GANgealing/models/spatial_transformers/spatial_transformer.py", line 5, in <module>
    from models.stylegan2.networks import EqualLinear, ConvLayer, ResBlock
  File "project-py-GANgealing/models/stylegan2/networks.py", line 6, in <module>
    from models.stylegan2.op import FusedLeakyReLU, fused_leaky_relu, upfirdn2d, conv2d_gradfix
  File "project-py-GANgealing/models/stylegan2/op/__init__.py", line 1, in <module>
    from .fused_act import FusedLeakyReLU, fused_leaky_relu
  File "project-py-GANgealing/models/stylegan2/op/fused_act.py", line 15, in <module>
    os.path.join(module_path, "fused_bias_act_kernel.cu"),
  File "python3.7/site-packages/torch/utils/cpp_extension.py", line 1156, in load
    keep_intermediates=keep_intermediates)
  File "python3.7/site-packages/torch/utils/cpp_extension.py", line 1367, in _jit_compile
    is_standalone=is_standalone)
  File "python3.7/site-packages/torch/utils/cpp_extension.py", line 1465, in _write_ninja_file_and_build_library
    is_standalone=is_standalone)
  File "python3.7/site-packages/torch/utils/cpp_extension.py", line 1857, in _write_ninja_file_to_build_library
    cuda_flags = common_cflags + COMMON_NVCC_FLAGS + _get_cuda_arch_flags()
  File "python3.7/site-packages/torch/utils/cpp_extension.py", line 1626, in _get_cuda_arch_flags
    arch_list[-1] += '+PTX'
IndexError: list index out of range