Prepare TC for the next release - May ?, 2018

[prigoyal]

In the next release, we want to improve the pybindings besides other things:

• [ ] PyTorch now provides the cpp extensions that we can use to extract aten tensor directly from PyTorch tensor. This should help with perf.
• [ ] Variable/Tensor are merged in PyTorch master/next release. TC python API needs to adapt accordingly.
• [x] There are some segfaults in TC cpp tests when pytorch conda package is used especially when used in CUDA 9.0 env.
• [ ] Build new conda packages for CUDA8/9
• [ ] Update docs to reflect API changes and new features