Add unit test for MNIST example tfjs translation

[vvmnnnkv]

Description

Add unit test that passes if MNIST example training Plan successfully translates to TFJS.
(It doesn't test correctness of translation, though, as it would require running JS engine with TFJS)

Affected Dependencies

This unit test should pass after these are merged:

• Fix Plan trace for inplace ops, add Role actions pruning #3798 - to remove .dim() that can't be translated yet but not needed in training plan
• Add tfjs translation commands for MNIST example Threepio#99 - adds more command translation rules in 3p0

How has this been tested?

It's unit test on its own.

Checklist

• I have followed the Contribution Guidelines and Code of Conduct
• I have commented my code following the OpenMined Styleguide
• I have labeled this PR with the relevant Type labels
• My changes are covered by tests

[Nolski]

@vvmnnnkv If you update to 3p0 0.0.10 this should resolve the 2nd issue you mentioned

[vvmnnnkv]

@Nolski
Looks like we need this fix: OpenMined/Threepio#109 in threepio.
I've used branch directly in requirements.txt to pass unit test in PySyft.
If threepio is published under new version, requirements.txt needs to be updated.

[codecov]

Codecov Report

Merging #3802 into master will increase coverage by 0.00%.
The diff coverage is 96.77%.

@@           Coverage Diff           @@
##           master    #3802   +/-   ##
=======================================
  Coverage   95.07%   95.07%           
=======================================
  Files         186      186           
  Lines       18857    18868   +11     
=======================================
+ Hits        17928    17939   +11     
  Misses        929      929           

Impacted Files	Coverage Δ
test/execution/test_plan.py	100.00% <ø> (ø)
...frameworks/torch/tensors/interpreters/gradients.py	94.19% <33.33%> (-0.04%)	⬇️
syft/execution/action.py	93.91% <100.00%> (+0.05%)	⬆️
syft/execution/plan.py	94.85% <100.00%> (+0.01%)	⬆️
test/execution/test_translation.py	100.00% <100.00%> (ø)