[Reformer] fix reformer num buckets
Fix automatic setting of `num_buckets` by making sure `num_buckets` is always a power of 2 and set as default.

The idea behind this whole function is that `num_buckets` should not be set by the user, but calculated on the fly to a good value before training (`num_buckets` ~ 2 * sequence length / chunk length as recommend in the paper). This value will then be saved in the config and can be reapplied for inference.
# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/4564?src=pr&el=h1) Report
> Merging [#4564](https://codecov.io/gh/huggingface/transformers/pull/4564?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/a34a9896ac2a4a33ff9cd805c76eed914c8d8965&el=desc) will **decrease** coverage by `0.01%`.
> The diff coverage is `14.28%`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/4564/graphs/tree.svg?width=650&height=150&src=pr&token=9qOlN6Hb1c)](https://codecov.io/gh/huggingface/transformers/pull/4564?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master    #4564      +/-   ##
==========================================
- Coverage   77.87%   77.86%   -0.02%     
==========================================
  Files         123      123              
  Lines       20566    20569       +3     
==========================================
  Hits        16016    16016              
- Misses       4550     4553       +3     
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/4564?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [src/transformers/configuration\_reformer.py](https://codecov.io/gh/huggingface/transformers/pull/4564/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9jb25maWd1cmF0aW9uX3JlZm9ybWVyLnB5) | `100.00% <ø> (ø)` | |
| [src/transformers/modeling\_reformer.py](https://codecov.io/gh/huggingface/transformers/pull/4564/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ19yZWZvcm1lci5weQ==) | `87.94% <14.28%> (-0.24%)` | :arrow_down: |
| [src/transformers/modeling\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/4564/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ191dGlscy5weQ==) | `90.41% <0.00%> (-0.12%)` | :arrow_down: |
| [src/transformers/modeling\_tf\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/4564/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl91dGlscy5weQ==) | `88.66% <0.00%> (ø)` | |

------

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> Powered by [Codecov](https://codecov.io/gh/huggingface/transformers/pull/4564?src=pr&el=footer). Last update [a34a989...7b40493](https://codecov.io/gh/huggingface/transformers/pull/4564?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
