Operators for StyleGAN2

All files in this directory are borrowed from repository stylegan2-ada-pytorch. Basically, these files implement customized operators, which are faster than the native operators from PyTorch, especially for second-derivative computation, including

bias_act.bias_act(): Fuse adding bias and then performing activation as one operator.
upfirdn2d.setup_filter(): Set up the kernel used for filtering.
upfirdn2d.filter2d(): Filtering a 2D feature map with given kernel.
upfirdn2d.upsample2d(): Upsampling a 2D feature map.
upfirdn2d.downsample2d(): Downsampling a 2D feature map.
upfirdn2d.upfirdn2d(): Upsampling, filtering, and then downsampling a 2D feature map.
conv2d_gradfix.conv2d(): Convolutional layer, supporting arbitrarily high order gradients and fixing gradient when computing penalty.
conv2d_gradfix.conv_transpose2d(): Transposed convolutional layer, supporting arbitrarily high order gradients and fixing gradient when computing penalty.
conv2d_resample.conv2d_resample(): Wraps upfirdn2d() and conv2d() (or conv_transpose2d()). This is not used in our network implementation (i.e., models/stylegan2_generator.py and models/stylegan2_discriminator.py)

We make following slight modifications beyond disabling some lint warnings:

Line 25 of file misc.py: Use EasyDict from module easydict to replace that from dnnlib from stylegan2-ada-pytorch.
Line 35 of file custom_ops.py: Disable log message when setting up customized operators.
Line 53/89 of file custom_ops.py: Add necessary CUDA compiler path. (NOTE: If your cuda binary does not locate at /usr/local/cuda/bin, please specify in function _find_compiler_bindir_posix().)
Line 24 of file bias_act.py: Use EasyDict from module easydict to replace that from dnnlib from stylegan2-ada-pytorch.
Line 32 of file grid_sample_gradfix.py: Enable customized grid sampling operator by default.
Line 36 of file grid_sample_gradfix.py: Use impl to disable customized grid sample operator.
Line 33 of file conv2d_gradfix.py: Enable customized convolution operators by default.
Line 46/51 of file conv2d_gradfix.py: Use impl to disable customized convolution operators.
Line 66 of file conv2d_gradfix.py: Update PyTorch version check considering the sustained development of the community.
Line 47 of file grid_sample_gradfix.py: Update PyTorch version check considering the sustained development of the community.
Line 36/66 of file conv2d_resample.py: Use impl to disable customized convolution operators.
Line 23 of file fma.py: Use impl to disable customized add-multiply operator.

Please use ref or cuda to choose which implementation to use. ref refers to native PyTorch operators while cuda refers to the customized operators from the official repository. cuda is used by default.