Speed up RepetitionPenaltyLogitsProcessor (pytorch)
# What does this PR do?
Speeds up RepetitionPenaltyLogitsProcessor using torch gather-scatter functions. Tested on pytorch 1.4.0.
Here's a minimal example to reproduce the slow behavior (and test speed of improvements):
```
import torch
from transformers import RepetitionPenaltyLogitsProcessor, LogitsProcessor
import timeit
import sys


class RepetitionPenaltyLogitsProcessorNew(LogitsProcessor):
    r"""
    :class:`transformers.LogitsProcessor` enforcing an exponential penalty on repeated sequences.

    Args:
        repetition_penalty (:obj:`float`):
            The parameter for repetition penalty. 1.0 means no penalty. See `this paper
            <https://arxiv.org/pdf/1909.05858.pdf>`__ for more details.
    """

    def __init__(self, penalty: float):
        if not isinstance(penalty, float) or not (penalty > 0):
            raise ValueError(f"`penalty` has to be a strictly positive float, but is {penalty}")

        self.penalty = penalty

    def __call__(self, input_ids: torch.LongTensor, scores: torch.FloatTensor) -> torch.FloatTensor:
        score = torch.gather(scores, 1, input_ids)  # changed here

        # if score < 0 then repetition penalty has to be multiplied to reduce the previous token probability
        score = torch.where(score < 0, score * self.penalty, score / self.penalty)

        scores.scatter_(1, input_ids, score)  # changed here
        return scores


input_ids = torch.randint(0, 10000, (256, 256))
scores = torch.randn(256, 10000)

rep_proc = RepetitionPenaltyLogitsProcessor(1.3)

rep_proc_new = RepetitionPenaltyLogitsProcessorNew(1.3)

assert torch.eq(rep_proc(input_ids, scores), rep_proc_new(input_ids, scores)).all().item(), "Should be equal"

print("Python version:", sys.version)
print("Pytorch version:", torch.__version__, "\n")

print(f"Existing rep_proc impl time for 100 iterations on CPU = {timeit.timeit(lambda: rep_proc(input_ids, scores), number=100)}")
print(f"Proposed rep_proc impl time for 100 iterations on CPU = {timeit.timeit(lambda: rep_proc_new(input_ids, scores), number=100)}\n")

if torch.cuda.is_available():
    input_ids = input_ids.cuda()
    scores = scores.cuda()

    print(f"Existing rep_proc impl time for 100 iterations on GPU = {timeit.timeit(lambda: rep_proc(input_ids, scores), number=100)}")
    print(f"Proposed rep_proc impl time for 100 iterations on GPU = {timeit.timeit(lambda: rep_proc_new(input_ids, scores), number=100)}")

```
Timings reported:
```
Python version: 3.7.9 (default, Aug 31 2020, 12:42:55) 
[GCC 7.3.0]
Pytorch version: 1.4.0 

Existing rep_proc impl time for 100 iterations on CPU = 0.0807734300001357
Proposed rep_proc impl time for 100 iterations on CPU = 0.044223628000054305

Existing rep_proc impl time for 100 iterations on GPU = 0.017542457000217837
Proposed rep_proc impl time for 100 iterations on GPU = 0.00720681400025569
```


## 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.
- [ ] 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?

@LysandreJik, @patrickvonplaten
 

