[v0.4] checklist to migrate to pytorch 0.4.0

[WEI HAORAN]

I list all the places need to be modified. Feel free to add below.

• Replace .data with .detach(). (#16, c3636b0)
• Update pytorch requirement and v0.3 branch in README.md. (2bbae4a)
• Use set_grad_enabled, enable_grad, no_grad as context-manager. (c3636b0)
• Use official LayerNorm instead. (247b3a3)
• Use clip_grad_norm_ instead. (247b3a3)
  Fix shard in v0.4

[WEI HAORAN]

Unfortunately, shard mechanism maybe not effective in PyTorch 0.4 version.