Added convenience function to load MNIST data as torch tensors

[mjjimenez]

Description

Added convenience function to load MNIST data as torch tensors

How has this been tested?

No unit tests, since it's only part of the sample project

Checklist

• [x] I have followed the Contribution Guidelines and Code of Conduct
• [x] I have commented my code following the OpenMined Styleguide
• [x] I have labeled this PR with the relevant Type labels

[codecov[bot]]

Codecov Report

Merging #184 into dev will decrease coverage by 6.98%. The diff coverage is 15.07%.

@@            Coverage Diff             @@
##              dev     #184      +/-   ##
==========================================
- Coverage   52.97%   45.98%   -6.99%     
==========================================
  Files          16       21       +5     
  Lines        1882     2318     +436     
==========================================
+ Hits          997     1066      +69     
- Misses        885     1252     +367     

Impacted Files	Coverage Δ
SwiftSyft/Classes/SyftPlan.swift	7.54% <0.00%> (ø)
SwiftSyft/Classes/TorchWrapper/src/TorchIValue.mm	0.00% <0.00%> (ø)
SwiftSyft/Classes/TorchWrapper/src/TorchModule.mm	0.00% <0.00%> (ø)
SwiftSyft/Classes/TorchWrapper/src/TorchTensor.mm	0.00% <0.00%> (ø)
SwiftSyft/Classes/DataLoader.swift	15.38% <15.38%> (ø)
SwiftSyft/Classes/SwiftSyftError.swift	23.80% <23.80%> (ø)
SwiftSyft/Classes/SignallingMessages.swift	35.03% <50.00%> (ø)
SwiftSyft/Classes/SyftClient.swift	82.64% <70.51%> (+3.92%)	:arrow_up:
SwiftSyft/Classes/NetworkManager.swift	73.07% <100.00%> (ø)
... and 7 more

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