sac example tensor size miss-match warning

[mtaohuang]

• [ ] I have marked all applicable categories:
  • [x] exception-raising bug
  • [ ] RL algorithm bug
  • [ ] documentation request (i.e. "X is missing from the documentation.")
  • [ ] new feature request
• [x] I have visited the [source website], and in particular read the [known issues]
• [x] I have searched through the [issue categories] for duplicates
• [x] I have mentioned version numbers, operating system and environment, where applicable:

  import tianshou, torch, sys
  print(tianshou.__version__, torch.__version__, sys.version, sys.platform)

• version numbers:

  0.2.2 1.4.0 3.7.3 (default, Mar 27 2019, 22:11:17) 
  [GCC 7.3.0] linux

• problem: UserWarning: Using a target size (torch.Size([128, 4])) that is different to the input size (torch.Size([128, 1])) Seems the shape of current_q1 and current_q2 is not correct.

  
  python3 examples/halfcheetahBullet_v0_sac.py --task BipedalWalkerHardcore-v3 --run-id train
  Epoch #1:   0%|                                                                                                                                                                    | 0/1000 [00:00<?, ?it/s]
  /usr/local/lib/python3.6/site-packages/tianshou/policy/modelfree/sac.py:111: UserWarning: Using a target size (torch.Size([128, 4])) that is different to the input size (torch.Size([128, 1])). This will likely lead to incorrect results due to broadcasting. Please ensure they have the same size.
  critic1_loss = F.mse_loss(current_q1, target_q)
  /usr/local/lib/python3.6/site-packages/tianshou/policy/modelfree/sac.py:117: UserWarning: Using a target size (torch.Size([128, 4])) that is different to the input size (torch.Size([128, 1])). This will likely lead to incorrect results due to broadcasting. Please ensure they have the same size.
  critic2_loss = F.mse_loss(current_q2, target_q)

  [source website]: https://github.com/thu-ml/tianshou/
  [known issues]: https://github.com/thu-ml/tianshou/#faq-and-known-issues
  [issue categories]: https://github.com/thu-ml/tianshou/projects/2

[Trinkle23897]

This is fixed in #41. I have just updated the issue categories.