[Reformer] fix default generators for pytorch < 1.6
As far as I know in PyTorch < 1.6 torch.cuda.default_generators is only defined when running on GPU => so we can simply add a `hasattr(torch.cuda, "default_generators")` to fix Reformer for PyTorch < 1.6.
Good to merge for me.<|||||># [Codecov](https://codecov.io/gh/huggingface/transformers/pull/6300?src=pr&el=h1) Report
> Merging [#6300](https://codecov.io/gh/huggingface/transformers/pull/6300?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/2f2aa0c89cab9a77560e6845578f917a61081c67&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/6300/graphs/tree.svg?width=650&height=150&src=pr&token=9qOlN6Hb1c)](https://codecov.io/gh/huggingface/transformers/pull/6300?src=pr&el=tree)

```diff
@@           Coverage Diff           @@
##           master    #6300   +/-   ##
=======================================
  Coverage   79.14%   79.15%           
=======================================
  Files         148      148           
  Lines       27191    27191           
=======================================
+ Hits        21521    21522    +1     
+ Misses       5670     5669    -1     
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/6300?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [src/transformers/modeling\_reformer.py](https://codecov.io/gh/huggingface/transformers/pull/6300/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ19yZWZvcm1lci5weQ==) | `95.68% <100.00%> (ø)` | |
| [src/transformers/generation\_tf\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/6300/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9nZW5lcmF0aW9uX3RmX3V0aWxzLnB5) | `86.71% <0.00%> (+0.25%)` | :arrow_up: |

------

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> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
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