[Reformer] Make random seed generator available on random seed and not on model device

[patrickvonplaten]

@LysandreJik - that might possibly fix the circle ci error.

[codecov]

Codecov Report

Merging #6244 into master will decrease coverage by 0.31%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #6244      +/-   ##
==========================================
- Coverage   79.61%   79.30%   -0.32%     
==========================================
  Files         146      146              
  Lines       26597    26595       -2     
==========================================
- Hits        21175    21090      -85     
- Misses       5422     5505      +83     

Impacted Files	Coverage Δ
src/transformers/configuration_encoder_decoder.py	100.00% <ø> (ø)
src/transformers/modeling_encoder_decoder.py	92.20% <ø> (ø)
src/transformers/benchmark/benchmark_args_utils.py	89.13% <100.00%> (ø)
src/transformers/modeling_reformer.py	95.68% <100.00%> (+0.19%)	⬆️
src/transformers/tokenization_utils_base.py	86.43% <100.00%> (-7.42%)	⬇️
src/transformers/modeling_tf_bert.py	70.11% <0.00%> (-26.82%)	⬇️
src/transformers/tokenization_roberta.py	76.71% <0.00%> (-21.92%)	⬇️
src/transformers/tokenization_transfo_xl.py	38.73% <0.00%> (-3.76%)	⬇️
src/transformers/tokenization_utils_fast.py	92.14% <0.00%> (-2.15%)	⬇️
... and 6 more

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[LysandreJik]

Tested on the machine, it works flawlessly! Thanks!