[Reformer] Axial Pos Emb Improve mem usage reformer
This PR improves memory usage of Axial Position Encodings by cutting position encodings only to the required length before applying contiguous pytorch operations.
# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/5209?src=pr&el=h1) Report
> Merging [#5209](https://codecov.io/gh/huggingface/transformers/pull/5209?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/355954ffca798bb81d9db8886e30ce10f11e8a40&el=desc) will **increase** coverage by `0.00%`.
> The diff coverage is `100.00%`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/5209/graphs/tree.svg?width=650&height=150&src=pr&token=9qOlN6Hb1c)](https://codecov.io/gh/huggingface/transformers/pull/5209?src=pr&el=tree)

```diff
@@           Coverage Diff           @@
##           master    #5209   +/-   ##
=======================================
  Coverage   77.28%   77.28%           
=======================================
  Files         133      133           
  Lines       22134    22135    +1     
=======================================
+ Hits        17107    17108    +1     
  Misses       5027     5027           
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/5209?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [src/transformers/modeling\_reformer.py](https://codecov.io/gh/huggingface/transformers/pull/5209/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ19yZWZvcm1lci5weQ==) | `88.21% <100.00%> (+0.01%)` | :arrow_up: |
| [src/transformers/file\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/5209/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9maWxlX3V0aWxzLnB5) | `75.00% <0.00%> (-0.41%)` | :arrow_down: |
| [src/transformers/trainer.py](https://codecov.io/gh/huggingface/transformers/pull/5209/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy90cmFpbmVyLnB5) | `39.62% <0.00%> (+0.23%)` | :arrow_up: |

------

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> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/huggingface/transformers/pull/5209?src=pr&el=footer). Last update [355954f...cd443f7](https://codecov.io/gh/huggingface/transformers/pull/5209?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
