backend updates

[DvirDukhan]

this PR

1. creates multi CUDA variant GPU docker with 11.0 and 10.2
2. updates Torchto 1.7.1 CUDA 11.0
3. updates TensorFlow to 2.4 CUDA 11.0
4. updates ONNX to 1.6 CUDA 10.2

closes #459

[codecov]

Codecov Report

Merging #566 (29f7c92) into master (a8695b9) will increase coverage by 1.59%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #566      +/-   ##
==========================================
+ Coverage   74.13%   75.72%   +1.59%     
==========================================
  Files          39       39              
  Lines        6081     6081              
==========================================
+ Hits         4508     4605      +97     
+ Misses       1573     1476      -97     

Impacted Files	Coverage Δ
src/DAG/dag.c	84.46% <0.00%> (+2.18%)	⬆️
src/backends/onnxruntime.c	67.47% <0.00%> (+6.72%)	⬆️
src/tensor.c	84.20% <0.00%> (+7.99%)	⬆️
src/backends/torch.c	80.17% <0.00%> (+8.62%)	⬆️

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