[ctrl] fix pruning of MultiHeadAttention
@sshleifer 
Implemented the pruning logic. Fixes - #4798 
After enabling `test_pruning` all the previously failing tests are passing.
Hi @aretius, some (if not all) the tests failing are unrelated to your PR, and should have been solved by the recently merged #4903. Do you mind rebasing on `master` and force pushing so that we may see if all the tests pass?<|||||># [Codecov](https://codecov.io/gh/huggingface/transformers/pull/4904?src=pr&el=h1) Report
> Merging [#4904](https://codecov.io/gh/huggingface/transformers/pull/4904?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/ac99217e92c43066af7ec96554054d75532565d7&el=desc) will **increase** coverage by `0.01%`.
> The diff coverage is `93.33%`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/4904/graphs/tree.svg?width=650&height=150&src=pr&token=9qOlN6Hb1c)](https://codecov.io/gh/huggingface/transformers/pull/4904?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master    #4904      +/-   ##
==========================================
+ Coverage   76.99%   77.01%   +0.01%     
==========================================
  Files         128      128              
  Lines       21602    21615      +13     
==========================================
+ Hits        16633    16647      +14     
+ Misses       4969     4968       -1     
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/4904?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [src/transformers/modeling\_ctrl.py](https://codecov.io/gh/huggingface/transformers/pull/4904/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ19jdHJsLnB5) | `99.14% <93.33%> (+0.50%)` | :arrow_up: |

------

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> Powered by [Codecov](https://codecov.io/gh/huggingface/transformers/pull/4904?src=pr&el=footer). Last update [ac99217...bf94b7e](https://codecov.io/gh/huggingface/transformers/pull/4904?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
<|||||>@sshleifer do you want to take a look?