Separate TFRecords code on different file

[JacopoMangiavacchi]

WIP: Not ready yet for approving

[codecov]

Codecov Report

Merging #514 into v2 will decrease coverage by 0.04%.
The diff coverage is 97.29%.

@@            Coverage Diff             @@
##               v2     #514      +/-   ##
==========================================
- Coverage   83.81%   83.76%   -0.05%     
==========================================
  Files         107      108       +1     
  Lines        3003     3000       -3     
  Branches      489      495       +6     
==========================================
- Hits         2517     2513       -4     
- Misses        486      487       +1

Impacted Files	Coverage Δ
...ders/export/tensorFlowRecords/tensorFlowHelpers.ts	98.14% <100%> (+0.23%)	⬆️
src/providers/export/tensorFlowRecords.ts	88.65% <100%> (-3.5%)	⬇️
...ders/export/tensorFlowRecords/tensorFlowBuilder.ts	95.74% <95.74%> (ø)

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[wbreza]

Looks good overall - Added some suggestions that may make the API easier to use.

[elizabethhalper]

Looks good! Once you address Wallace's comments I think it should be good to go