tensor len member

[DvirDukhan]

this PR adds a new len member to RAI_Tensor which is lazy evaluated on RAI_TensorLength
This should reduce the amount of calls to RAI_TensorLength

closes #495

[codecov]

Codecov Report

Merging #581 (68534d6) into master (14e82db) will increase coverage by 0.98%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #581      +/-   ##
==========================================
+ Coverage   74.33%   75.32%   +0.98%     
==========================================
  Files          39       39              
  Lines        6063     6062       -1     
==========================================
+ Hits         4507     4566      +59     
+ Misses       1556     1496      -60     

Impacted Files	Coverage Δ
src/backends/onnxruntime.c	60.75% <100.00%> (-0.11%)	⬇️
src/backends/tensorflow.c	70.50% <100.00%> (-0.09%)	⬇️
...c/serialization/RDB/decoder/current/v1/decode_v1.c	64.63% <100.00%> (-0.22%)	⬇️
.../serialization/RDB/decoder/previous/v0/decode_v0.c	61.39% <100.00%> (-0.25%)	⬇️
src/tensor.c	84.64% <100.00%> (+8.18%)	⬆️
src/backends/torch.c	78.96% <0.00%> (+7.29%)	⬆️

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[DvirDukhan]

@filipecosta90 Can we benchmark before merge?

[lantiga]

LGTM