Fixed horizon_length for PPLM
# What does this PR do?
The current code can only run at horizon_length=1.
However, It's not applicable if we want to run at horizon_length > 1 since the curr_probs is not updated in the for_loop.
This PR is to update the curr_probs in the horizon_length in the for_loop.

Fixes # (issue)


## Before submitting
- [ ] This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
- [x] Did you read the [contributor guideline](https://github.com/huggingface/transformers/blob/master/CONTRIBUTING.md#start-contributing-pull-requests),
      Pull Request section?
- [ ] Was this discussed/approved via a Github issue or the [forum](https://discuss.huggingface.co/)? Please add a link
      to it if that's the case.
- [x] Did you make sure to update the documentation with your changes? Here are the
      [documentation guidelines](https://github.com/huggingface/transformers/tree/master/docs), and
      [here are tips on formatting docstrings](https://github.com/huggingface/transformers/tree/master/docs#writing-source-documentation).
- [ ] Did you write any new necessary tests?


## Who can review?

Anyone in the community is free to review the PR once the tests have passed. Feel free to tag
members/contributors who may be interested in your PR.

@sgugger sorry for bothering you, could you help me review my PR?
Hope to see your review, thanks so much!<|||||>Hi @jacksukk, thanks for your PR but as mentioned in the [README](https://github.com/huggingface/transformers/tree/master/examples/research_projects) of the research projects, you need to tag the author of the project. We don't maintain that code ourselves.<|||||>Hi @sgugger, Thanks a lot for your reply! QQ

Let me tag @dathath, could you help to review my code? 
Sorry for bothering and hope to see your comment. Thanks!<|||||>Hi @jacksukk,
Thanks for the ping! I've seen this, and will review shortly.<|||||>Thanks a lot both!<|||||>Thanks a lot! I'm a big fan of both huggingface and PPLM. 
I'm delighted to have some commit here!